patch 9.0.0223: typo in diffmode test

Problem:    Typo in diffmode test.
Solution:   Fix the typo. (closes #10932)
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index 41f7fe3..3c9fbec 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
Binary files differ