maple_tree: fix outdated flag name in comment

MAPLE_USE_RCU was renamed to MT_FLAGS_USE_RCU at some point, fix up the
comment.

Link: https://lkml.kernel.org/r/20241007-maple-tree-doc-fix-v1-1-6bbf89c1153d@google.com


Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>