docs: remove reference to non-existent "igmp-ipv6" protocol

IGMP is used on IPv4 networks tp setup multicast group memberships. On
IPv6, this job is done by Multicast Listener Discovery (MLD), which
uses ICMPv6 packets rather than its own IP protocol number like IGMP.

The nwfilter documentation lists "igmp-ipv6" as one of the possible
protocols, but this is ignored (and stripped from the xml). This patch
removes that erroneous reference.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1038888

docs/formatnwfilter.html.in

@@ -1781,7 +1781,7 @@
 
     <h5><a name="nwfelemsRulesProtoMiscv6">IGMP, ESP, AH, UDPLITE, 'ALL' over IPv6</a></h5>
     <p>
-      Protocol ID: <code>igmp-ipv6</code>, <code>esp-ipv6</code>, <code>ah-ipv6</code>, <code>udplite-ipv6</code>, <code>all-ipv6</code>
+      Protocol ID: <code>esp-ipv6</code>, <code>ah-ipv6</code>, <code>udplite-ipv6</code>, <code>all-ipv6</code>
       <br/>
       Note: The chain parameter is ignored for this type of traffic
       and should either be omitted or set to <code>root</code>.