Minor C API documentation improvements. (GH-17697)

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
(cherry picked from commit 5c7ed7550ec2da16d7679e538fcd7c1a5631811f)

Co-authored-by: William Ayd <william.ayd@icloud.com>

2 files changed, 2 insertions, 2 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 */
diff --git a/Doc/includes/custom.c b/Doc/includes/custom.c
index bda32e2..f361baf 100644
--- a/Doc/includes/custom.c
+++ b/Doc/includes/custom.c
@@ -37,7 +37,7 @@ PyInit_custom(void)
     Py_INCREF(&CustomType);
     if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
         Py_DECREF(&CustomType);
-        PY_DECREF(m);
+        Py_DECREF(m);
         return NULL;
     }