patch 8.1.1838: there is :spellwrong and :spellgood but not :spellrare

Problem:    There is :spellwrong and :spellgood but not :spellrare.
Solution:   Add :spellrare. (Martin Tournoij, closes #4291)
diff --git a/src/proto/spellfile.pro b/src/proto/spellfile.pro
index 70e993a..552582a 100644
--- a/src/proto/spellfile.pro
+++ b/src/proto/spellfile.pro
@@ -5,5 +5,5 @@
 void ex_mkspell(exarg_T *eap);
 void mkspell(int fcount, char_u **fnames, int ascii, int over_write, int added_word);
 void ex_spell(exarg_T *eap);
-void spell_add_word(char_u *word, int len, int bad, int idx, int undo);
+void spell_add_word(char_u *word, int len, int what, int idx, int undo);
 /* vim: set ft=c : */