Minor C API documentation improvements. (GH-17696)

The added parentheses around the PyIter_Next assignment suppress the following warning which gcc throws without:
```
warning: using the result of an assignment as a condition without parentheses [-Wparentheses]
```
The other change is a typo fix

1 files changed, 1 insertions, 1 deletions

diff --git a/Doc/c-api/iter.rst b/Doc/c-api/iter.rst
index 0224d37..a2992b3 100644
--- a/Doc/c-api/iter.rst
+++ b/Doc/c-api/iter.rst
@@ -29,7 +29,7 @@ something like this::
        /* propagate error */
    }
 
-   while (item = PyIter_Next(iterator)) {
+   while ((item = PyIter_Next(iterator))) {
        /* do something with item */
        ...
        /* release reference when done */