tools: snmp: fix 'minimal snmp configuration' section in conf file

key name has to be FILTER and not FILTERS. users using current example
will not get any snmp notification.

Change-Id: I0202abfd9c2d26e41d2073a293e27d983f7578f9
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1072849
Signed-off-by: Mooli Tayer <mtayer@redhat.com>