Fixed a few obvious mistakes in c-api docs (GH-11184)

I thought these simple changes doesn't need bpo number(Am I right..?).

Please refer to the commit message for detail.

Doc/c-api/intro.rst

@@ -234,7 +234,7 @@ duration of the call.
 However, a common pitfall is to extract an object from a list and hold on to it
 for a while without incrementing its reference count. Some other operation might
 conceivably remove the object from the list, decrementing its reference count
-and possible deallocating it. The real danger is that innocent-looking
+and possibly deallocating it. The real danger is that innocent-looking
 operations may invoke arbitrary Python code which could do this; there is a code
 path which allows control to flow back to the user from a :c:func:`Py_DECREF`, so
 almost any operation is potentially dangerous.

Doc/c-api/refcounting.rst

@@ -58,8 +58,8 @@ objects.
    the macro carefully uses a temporary variable and sets the argument to *NULL*
    before decrementing its reference count.
 
-   It is a good idea to use this macro whenever decrementing the value of a
-   variable that might be traversed during garbage collection.
+   It is a good idea to use this macro whenever decrementing the reference
+   count of an object that might be traversed during garbage collection.
 
 
 The following functions are for runtime dynamic embedding of Python: