Documentation: fix netdev-FAQ.rst markup warning

Fix ReST underline warning:

./Documentation/networking/netdev-FAQ.rst:135: WARNING: Title underline too short.

Q: I made changes to only a few patches in a patch series should I resend only those changed?
--------------------------------------------------------------------------------------------

Fixes: ffa91253 ("Documentation: networking: Update netdev-FAQ regarding patches")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>