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_diffmode.vim b/src/testdir/test_diffmode.vim
index e8bd087..9338fe0 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
@@ -274,7 +274,7 @@
 endfunc
 
 " :diffput and :diffget completes names of buffers which
-" are in diff mode and which are different then current buffer.
+" are in diff mode and which are different than current buffer.
 " No completion when the current window is not in diff mode.
 func Test_diffget_diffput_completion()
   e            Xdiff1 | diffthis
@@ -677,7 +677,7 @@
   call assert_notequal(normattr, screenattr(3, 1))
   diffoff!
 
-  " Try using an non-existing function for 'diffexpr'.
+  " Try using a non-existing function for 'diffexpr'.
   set diffexpr=NewDiffFunc()
   call assert_fails('windo diffthis', ['E117:', 'E97:'])
   diffoff!