docs/core-api/mm: fix GFP combinations section name

Fix the mismatch between "Useful GFP flag combinations" section naming in
the DOC: section in include/linux/gfp.h and
Documentation/core-api/mm-api.rst.  This brings in the documentation,
and eliminates one warning:

    ./include/linux/gfp.h:1: warning: no structured comments found

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
[jc: tweaked changelog]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>