maple_tree: fix comment typo of ma_root

In comment of mas_start(), we lists the return value for different cases. 
In case of a single entry, we set mas->status to ma_root, while the
comment uses mas_root, which is not a maple_status.

Fix the typo according to the code.

Link: https://lkml.kernel.org/r/20240812150925.31551-1-richard.weiyang@gmail.com


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