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: 80cc4950 ("net: Add part of TCP counts explanations in snmp_counters.rst")
Fixes: 8e2ea53a ("add snmp counters document")
Fixes: a6c7c7aa ("net: add document for several snmp counters")

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: yupeng <yupeng0921@gmail.com>