c-user: Fix typos

Close #3261.

c-user/message_manager.rst

@@ -433,9 +433,9 @@ CALLING SEQUENCE:
     .. code-block:: c
 
         rtems_status_code rtems_message_queue_send(
-            rtems_id   id,
+            rtems_id    id,
             const void *buffer,
-            size_t     size
+            size_t      size
         );
 
 DIRECTIVE STATUS CODES:

c-user/rtems_data_types.rst

@@ -254,9 +254,9 @@ alphabetical order:
 ``rtems_single``
   This type is deprecated will be removed in RTEMS 6.1.  Use ``float`` instead.
 
-.. index:: rtems_status_codes
+.. index:: rtems_status_code
 
-``rtems_status_codes``
+``rtems_status_code``
   The return type for most RTEMS services.  This is an enumerated type of
   approximately twenty-five values.  In general, when a service returns a
   particular status code, it indicates that a very specific error condition has