patch 8.2.3914: various spelling mistakes in comments

Problem:    Various spelling mistakes in comments.
Solution:   Fix the mistakes. (Dominique Pellé, closes #9416)
diff --git a/src/testdir/test_source.vim b/src/testdir/test_source.vim
index 559208a..8f8e963 100644
--- a/src/testdir/test_source.vim
+++ b/src/testdir/test_source.vim
@@ -69,7 +69,7 @@
   call delete('Xfile.vim')
 endfunc
 
-" Test for expanding <sfile> in a autocmd and for <slnum> and <sflnum>
+" Test for expanding <sfile> in an autocmd and for <slnum> and <sflnum>
 func Test_source_autocmd_sfile()
   let code =<< trim [CODE]
     let g:SfileName = ''