doc: fix MLD route map documentation

Replace `IGMP` with `MLD`, this page is about IPv6 and not IPv4.

Signed-off-by: Rafael Zalamena <[email protected]>

Author: rzalamena

doc/user/pimv6.rst

@@ -319,7 +319,7 @@ is in a vrf, enter the interface command with the vrf keyword at the end.
 
 .. clicmd:: ipv6 mld route-map ROUTE-MAP
 
-   Apply the indicated route map to filter incoming IGMP joins.
+   Apply the indicated route map to filter incoming MLD joins.
 
    The following match statements can be used: