patch 9.0.0616: spell test fails because error message changed

Problem:    Spell test fails because error message changed.
Solution:   Adjust expected error message.
diff --git a/src/testdir/test_spell.vim b/src/testdir/test_spell.vim
index 3712b54..97c7e3c 100644
--- a/src/testdir/test_spell.vim
+++ b/src/testdir/test_spell.vim
@@ -147,7 +147,7 @@
   augroup TestSpellFileMissing
     autocmd! SpellFileMissing * bwipe
   augroup END
-  call assert_fails('set spell spelllang=ab_cd', 'E797:')
+  call assert_fails('set spell spelllang=ab_cd', 'E937:')
 
   " clean up
   augroup TestSpellFileMissing