Fix typo

Backends don't typically exist uncleanly, but they can certainly exit
uncleanly, and it's exiting uncleanly that's being discussed here.

src/backend/libpq/be-secure.c

@@ -180,7 +180,7 @@ retry:
 		/*
 		 * If the postmaster has died, it's not safe to continue running,
 		 * because it is the postmaster's job to kill us if some other backend
-		 * exists uncleanly.  Moreover, we won't run very well in this state;
+		 * exits uncleanly.  Moreover, we won't run very well in this state;
 		 * helper processes like walwriter and the bgwriter will exit, so
 		 * performance may be poor.  Finally, if we don't exit, pg_ctl will be
 		 * unable to restart the postmaster without manual intervention, so no