Fix type confusion in AddRelsyncInvalidationMessage

Fix type confusion in AddRelsyncInvalidationMessage

Since this is trying to add a SharedInvalRelSyncMsg rather than
a SharedInvalRelcacheMsg, it should use rs rather than rc.

This makes no difference as things stand, because the two structure
definitions are identical (except for the capitalization of "relid"),
but it's still a good idea to fix it.

Co-authored-by: Stolpovskikh Danil <d.stolpovskikh@ftdata.ru>
Co-authored-by: Robert Haas <rhaas@postgresql.org>
Discussion: http://postgr.es/m/bd6a5735b72b4afe99af49c3c62901d6@localhost.localdomain