Improve comment in function GetPublicationRelations

Hi,

While working on the thread [1], we found that the comment in the
function GetPublicationRelations is not correct. This function gets
the list of relations associated with the publication, but the comment
says the opposite :
/* Find all publications associated with the relation. */

Attached a patch to fix it.

[1]: https://www.postgresql.org/message-id/CANhcyEV_EVi5cgJ6WPvmeVAqjCS7Of%2BVAWuRHZtsVf8PQb_z7g%40mail.gmail.com

Thanks,
Shlok Kyal