Documentation: maple_tree: fix word spelling error

The "first" is spelled "fist".

Link: https://lkml.kernel.org/r/20231023095737.21823-1-yangqixiao@inspur.com


Signed-off-by: Tom Yang <yangqixiao@inspur.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>