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_spellfile.vim b/src/testdir/test_spellfile.vim
index 3bb42a5..38d1ec0 100644
--- a/src/testdir/test_spellfile.vim
+++ b/src/testdir/test_spellfile.vim
@@ -702,7 +702,7 @@
   let output = execute('mkspell! Xtest.spl Xtest')
   call assert_match('Different combining flag in continued affix block in Xtest.aff line 3', output)
 
-  " Try to reuse a affix used for BAD flag
+  " Try to reuse an affix used for BAD flag
   call writefile(['BAD x', 'PFX x Y 1', 'PFX x 0 re x'], 'Xtest.aff')
   let output = execute('mkspell! Xtest.spl Xtest')
   call assert_match('Affix also used for BAD/RARE/KEEPCASE/NEEDAFFIX/NEEDCOMPOUND/NOSUGGEST in Xtest.aff line 2: x', output)