Update expected stderr for affected tests, which are not under Tc directory

testsuite/tests/indexed-types/should_fail/T4485.stderr

@@ -22,7 +22,7 @@ T4485.hs:51:26: error:
     • Ambiguous type variable ‘m0’ arising from a use of ‘genElement’
       prevents the constraint ‘(XMLGen m0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘m0’ should be.
-      These potential instance exist:
+      This potential instance exists:
         instance XMLGen (IdentityT m) -- Defined at T4485.hs:38:10
     • In the second argument of ‘($)’, namely ‘(genElement "foo")’
       In the expression: asChild $ (genElement "foo")

testsuite/tests/rebindable/rebindable6.stderr

@@ -10,7 +10,7 @@ rebindable6.hs:110:17: error:
         test_do :: IO a -> IO (Maybe b) -> IO b
           (bound at rebindable6.hs:108:9)
       Probable fix: use a type annotation to specify what ‘t0’ should be.
-      These potential instance exist:
+      This potential instance exists:
         instance HasSeq (IO a -> IO b -> IO b)
           -- Defined at rebindable6.hs:56:18
     • In a stmt of a 'do' block: f
@@ -35,7 +35,7 @@ rebindable6.hs:111:17: error:
           (bound at rebindable6.hs:108:9)
       Probable fix: use a type annotation to specify what ‘p0’,
                                                           ‘t0’ should be.
-      These potential instance exist:
+      This potential instance exists:
         instance HasBind (IO a -> (a -> IO b) -> IO b)
           -- Defined at rebindable6.hs:51:18
     • In a stmt of a 'do' block: Just (b :: b) <- g
@@ -59,7 +59,7 @@ rebindable6.hs:112:17: error:
         test_do :: IO a -> IO (Maybe b) -> IO b
           (bound at rebindable6.hs:108:9)
       Probable fix: use a type annotation to specify what ‘p0’ should be.
-      These potential instance exist:
+      This potential instance exists:
         instance HasReturn (a -> IO a) -- Defined at rebindable6.hs:46:18
     • In a stmt of a 'do' block: return b
       In the expression: