Docs/core-api: fix typos in rbtree.rst

Docs/core-api: fix typos in rbtree.rst

Correct minor typographical errors in the red-black tree documentation:
- Remove redundant "a" in the cached rbtrees section.
- Fix "updated" to "update" in the augmented rbtrees section.
- Fix "be looking" to "by looking" in the interval tree sample usage.

Signed-off-by: Min-Hsun Chang <chmh0624@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260210060829.42975-1-chmh0624@gmail.com>