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

Documentation/networking/rds.rst

 .. SPDX-License-Identifier: GPL-2.0
 
-==
+===
 RDS
 ===