Doc: use const& in foreach when applicable.

Const reference are slightly faster than doing a copy of the object
on each iteration.

Lead by example by having this pattern right in our documentation

1 files changed, 2 insertions, 2 deletions

diff --git a/doc/src/snippets/picture/picture.cpp b/doc/src/snippets/picture/picture.cpp
index 0c5712a..5283978 100644
--- a/doc/src/snippets/picture/picture.cpp
+++ b/doc/src/snippets/picture/picture.cpp
@@ -76,7 +76,7 @@ int main()
         // FORMATS
 //! [2]
         QStringList list = QPicture::inputFormatList();
-        foreach (QString string, list)
+        foreach (const QString &string, list)
             myProcessing(string);
 //! [2]
     }
@@ -85,7 +85,7 @@ int main()
         // OUTPUT
 //! [3]
         QStringList list = QPicture::outputFormatList();
-        foreach (QString string, list)
+        foreach (const QString &string, list)
             myProcessing(string);
 //! [3]
     }