patch 8.1.1401: misspelled mkspellmem as makespellmem

Problem:    Misspelled mkspellmem as makespellmem.
Solution:   Drop duplicate help entry, fix test. (Naruhiko Nishino, Ken
            Takata, closes #4437)
diff --git a/src/testdir/test_modeline.vim b/src/testdir/test_modeline.vim
index fb3628b..21ac1e9 100644
--- a/src/testdir/test_modeline.vim
+++ b/src/testdir/test_modeline.vim
@@ -116,7 +116,7 @@
   call s:modeline_fails('luadll', 'luadll=Something()', 'E520:')
   call s:modeline_fails('makeef', 'makeef=Something()', 'E520:')
   call s:modeline_fails('makeprg', 'makeprg=Something()', 'E520:')
-  call s:modeline_fails('makespellmem', 'makespellmem=Something()', 'E520:')
+  call s:modeline_fails('mkspellmem', 'mkspellmem=Something()', 'E520:')
   call s:modeline_fails('mzschemedll', 'mzschemedll=Something()', 'E520:')
   call s:modeline_fails('mzschemegcdll', 'mzschemegcdll=Something()', 'E520:')
   call s:modeline_fails('modelineexpr', 'modelineexpr', 'E520:')