Message improvement

The previous wording contained a superfluous comma.  Adjust phrasing
for grammatical correctness and clarity.

contrib/test_decoding/expected/slot.out

@@ -144,7 +144,7 @@ SELECT pg_replication_slot_advance('regression_slot3', '0/0'); -- invalid LSN
 ERROR:  invalid target WAL LSN
 SELECT pg_replication_slot_advance('regression_slot3', '0/1'); -- error
 ERROR:  replication slot "regression_slot3" cannot be advanced
-DETAIL:  This slot has never previously reserved WAL, or has been invalidated.
+DETAIL:  This slot has never previously reserved WAL, or it has been invalidated.
 SELECT pg_drop_replication_slot('regression_slot3');
  pg_drop_replication_slot 
 --------------------------

src/backend/replication/logical/logicalfuncs.c

@@ -250,7 +250,7 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 					 errmsg("can no longer get changes from replication slot \"%s\"",
 							NameStr(*name)),
-					 errdetail("This slot has never previously reserved WAL, or has been invalidated.")));
+					 errdetail("This slot has never previously reserved WAL, or it has been invalidated.")));
 
 		MemoryContextSwitchTo(oldcontext);
 

src/backend/replication/slotfuncs.c

@@ -647,7 +647,7 @@ pg_replication_slot_advance(PG_FUNCTION_ARGS)
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 errmsg("replication slot \"%s\" cannot be advanced",
 						NameStr(*slotname)),
-				 errdetail("This slot has never previously reserved WAL, or has been invalidated.")));
+				 errdetail("This slot has never previously reserved WAL, or it has been invalidated.")));
 
 	/*
 	 * Check if the slot is not moving backwards.  Physical slots rely simply