mm: docs: Fix a comment in process_vm_rw_core

This removes a duplicate "a" in the comment in process_vm_rw_core.

Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>