bpo-38600: NULL -> ``NULL``. (GH-17001)

Also fix some other formatting.
diff --git a/Doc/c-api/import.rst b/Doc/c-api/import.rst
index ee09876..c6fc330 100644
--- a/Doc/c-api/import.rst
+++ b/Doc/c-api/import.rst
@@ -207,8 +207,8 @@
 .. c:function:: PyObject* PyImport_GetModule(PyObject *name)
 
    Return the already imported module with the given name.  If the
-   module has not been imported yet then returns NULL but does not set
-   an error.  Returns NULL and sets an error if the lookup failed.
+   module has not been imported yet then returns ``NULL`` but does not set
+   an error.  Returns ``NULL`` and sets an error if the lookup failed.
 
    .. versionadded:: 3.7