maple_tree: mtree_insert: fix typo in kernel-doc description of GFP flags

Replace FGP_FLAGS with GFP_FLAGS

Link: https://lkml.kernel.org/r/20230715084038.987955-1-rppt@kernel.org


Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>