patch 8.2.3887: E1135 is used for two different errors

Problem:    E1135 is used for two different errors.
Solution:   Renumber one error.
diff --git a/src/testdir/test_mapping.vim b/src/testdir/test_mapping.vim
index d2b9f37..dd0c8d1 100644
--- a/src/testdir/test_mapping.vim
+++ b/src/testdir/test_mapping.vim
@@ -905,7 +905,7 @@
   call assert_equal(0, x)
 
   noremap <F3> <Cmd>let x = 3
-  call assert_fails('call feedkeys("\<F3>", "xt!")', 'E1135:')
+  call assert_fails('call feedkeys("\<F3>", "xt!")', 'E1255:')
   call assert_equal(0, x)
 
   " works in various modes and sees the correct mode()