net-next: docs: Fix typos in snmp_counter.rst

This patch fixes some spelling typos in snmp_counter.rst

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>