docs: rbtree.rst: Fix a typo

s/maintanence/maintenance/

Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20210324080046.20709-1-unixbhaskar@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>