patch 8.2.3637: typos in test files

Problem:    Typos in test files.
Solution:   Correct the typos. (Dominique Pellé, closes #9175)
diff --git a/src/testdir/test_python2.vim b/src/testdir/test_python2.vim
index 89afa2b..3fc3c20 100644
--- a/src/testdir/test_python2.vim
+++ b/src/testdir/test_python2.vim
@@ -412,7 +412,7 @@
   py d = vim.bindeval('d')
   call assert_equal(2, pyeval('len(d)'))
 
-  " Deleting an non-existing key
+  " Deleting a non-existing key
   call AssertException(["py del d['c']"], "Vim(python):KeyError: 'c'")
 endfunc