Doc: networking: Fix the title's Sphinx overline in rds.rst

A missing "=" caused the title and sections to not show up properly in
the htmldocs.

Signed-off-by: Ahmed Zaki <anzaki@gmail.com>
Link: https://lore.kernel.org/r/20211128171719.3286255-1-anzaki@gmail.com


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