Fix typos identified using codespell

See https://github.com/codespell-project/codespell#readme

The following command was used:

```
codespell -q6 --skip="\
.git,\
*.json,\
./Copyright.txt,\
./Help/command/foreach.rst,\
./Help/prop_test/REQUIRED_FILES.rst,\
./Help/variable/CTEST_COVERAGE_COMMAND.rst,\
./Modules/CMakeCheckCompilerFlagCommonPatterns.cmake,\
./Modules/CMakeRCInformation.cmake,\
./Modules/Internal/CPack/NSIS.template.in,\
./Modules/FindMatlab.cmake,\
./Modules/MatlabTestsRedirect.cmake,\
./Modules/Platform/Windows-Clang.cmake,\
./Modules/Platform/Windows-Intel-Fortran.cmake,\
./Modules/Platform/Windows-MSVC.cmake,\
./Source/CMakeVersion.cmake,\
./Source/cmConvertMSBuildXMLToJSON.py,\
./Source/cmCreateTestSourceList.cxx,\
./Source/cmGlobalVisualStudio10Generator.cxx,\
./Source/cmExportBuildFileGenerator.cxx,\
./Source/cmExportInstallAndroidMKGenerator.cxx,\
./Source/cmExportInstallFileGenerator.cxx,\
./Source/cmExportSet.cxx,\
./Source/cmExportTryCompileFileGenerator.cxx,\
./Source/cmFindPackageCommand.cxx,\
./Source/cmInstallCommand.cxx,\
./Source/cmGeneratorExpressionLexer.cxx,\
./Source/cmLocalVisualStudio7Generator.cxx,\
./Source/cmOrderDirectories.cxx,\
./Source/cmTarget.cxx,\
./Source/kwsys/*,\
./Source/QtDialog/CMakeSetupDialog.ui,\
./Source/CPack/WiX/cmWIXRichTextFormatWriter.cxx,\
./Source/CTest/cmParseCoberturaCoverage.h,\
./Tests/CMakeTests/ImplicitLinkInfoTest.cmake.in,\
./Tests/RunCMake/CPack/tests/DMG_SLA/English.license.rtf,\
./Tests/RunCMake/CPack/tests/DMG_SLA/German.license.txt,\
./Tests/RunCMake/CPack/tests/DMG_SLA/German.menu.txt,\
./Tests/RunCMake/GoogleTest/xml_output.cpp,\
./Tests/RunCMake/Make/TargetMessages*,\
./Utilities/*,\
" \
-L "\
dependees,\
endwhile,\
fo,\
filetest,\
helpfull,\
nd,\
objext,\
stoll,\
supercedes,\
superceded,\
vas,\
varn,\
"
```

1 files changed, 2 insertions, 2 deletions

diff --git a/Source/CursesDialog/form/frm_driver.c b/Source/CursesDialog/form/frm_driver.c
index e4e72aa..112ab08 100644
--- a/Source/CursesDialog/form/frm_driver.c
+++ b/Source/CursesDialog/form/frm_driver.c
@@ -2983,7 +2983,7 @@ INLINE static FIELD *Right_Neighbour_Field(FIELD * field)
 |   Function      :  static FIELD *Upper_Neighbour_Field(FIELD * field)
 |   
 |   Description   :  Because of the row-major nature of sorting the fields,
-|                    its more difficult to define whats the upper neighbour
+|                    its more difficult to define what's the upper neighbour
 |                    field really means. We define that it must be on a
 |                    'previous' line (cyclic order!) and is the rightmost
 |                    field laying on the left side of the given field. If
@@ -3030,7 +3030,7 @@ static FIELD *Upper_Neighbour_Field(FIELD * field)
 |   Function      :  static FIELD *Down_Neighbour_Field(FIELD * field)
 |   
 |   Description   :  Because of the row-major nature of sorting the fields,
-|                    its more difficult to define whats the down neighbour
+|                    its more difficult to define what's the down neighbour
 |                    field really means. We define that it must be on a
 |                    'next' line (cyclic order!) and is the leftmost
 |                    field laying on the right side of the given field. If