patch 8.1.1951: mouse double click test is a bit flaky

Problem:    Mouse double click test is a bit flaky.
Solution:   Add to list of flaky tests.  Update a couple of comments.
diff --git a/src/testdir/test_substitute.vim b/src/testdir/test_substitute.vim
index 344bb68..a263fb6 100644
--- a/src/testdir/test_substitute.vim
+++ b/src/testdir/test_substitute.vim
@@ -504,7 +504,7 @@
   call Run_SubCmd_Tests(tests)
 endfunc
 
-" Test for submatch() on :substitue.
+" Test for submatch() on :substitute.
 func Test_sub_cmd_4()
   set magic&
   set cpo&