Fix grammar in error message

While at it, make ellipses formatting consistent when describing SQL statements.

Ekaterina Kiryanova and Alexander Lakhin

Reviewed by myself and Álvaro Herrera
Discussion: https://www.postgresql.org/message-id/eed5cec0-a542-53da-6a5e-7789c6ed9817%40postgrespro.ru
Backpatch only the grammar fix to v15

src/backend/postmaster/bgworker.c

@@ -663,7 +663,7 @@ SanityCheckBackgroundWorker(BackgroundWorker *worker, int elevel)
 	{
 		ereport(elevel,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("background worker \"%s\": background worker without shared memory access are not supported",
+				 errmsg("background worker \"%s\": background workers without shared memory access are not supported",
 						worker->bgw_name)));
 		return false;
 	}