networking: fix snmp_counter.rst Doc. Warnings

networking: fix snmp_counter.rst Doc. Warnings

Fix documentation markup warnings in snmp_counter.rst:

Documentation/networking/snmp_counter.rst:416: WARNING: Title underline too short.
Documentation/networking/snmp_counter.rst:684: WARNING: Bullet list ends without a blank line; unexpected unindent.
Documentation/networking/snmp_counter.rst:693: WARNING: Title underline too short.
Documentation/networking/snmp_counter.rst:707: WARNING: Bullet list ends without a blank line; unexpected unindent.
Documentation/networking/snmp_counter.rst:712: WARNING: Bullet list ends without a blank line; unexpected unindent.
Documentation/networking/snmp_counter.rst:722: WARNING: Title underline too short.
Documentation/networking/snmp_counter.rst:733: WARNING: Bullet list ends without a blank line; unexpected unindent.
Documentation/networking/snmp_counter.rst:736: WARNING: Bullet list ends without a blank line; unexpected unindent.
Documentation/networking/snmp_counter.rst:739: WARNING: Bullet list ends without a blank line; unexpected unindent.

Fixes: 945a701b69785 ("net: Add part of TCP counts explanations in snmp_counters.rst")
Fixes: 0a55553b71097 ("add snmp counters document")
Fixes: 7fa9a98f05d23 ("net: add document for several snmp counters")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: yupeng <yupeng0921@gmail.com>