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_syntax.vim b/src/testdir/test_syntax.vim
index 0a0d667..83a63c4 100644
--- a/src/testdir/test_syntax.vim
+++ b/src/testdir/test_syntax.vim
@@ -502,8 +502,7 @@
 func Test_bg_detection()
   CheckNotGui
 
-  " auto-detection of &bg, make sure sure it isn't set anywhere before
-  " this test
+  " auto-detection of &bg, make sure it isn't set anywhere before this test
   hi Normal ctermbg=0
   call assert_equal('dark', &bg)
   hi Normal ctermbg=4