spell.c: give priority to rare words. #2456

See:
  https://groups.google.com/forum/#!topic/vim_dev/rPWOoR3ZgSA

runtime/doc/spell.txt

@@ -1181,8 +1181,7 @@ rare words.  Example:
 
 Rare words are highlighted differently from bad words.  This is to be used for
 words that are correct for the language, but are hardly ever used and could be
-a typing mistake anyway.  When the same word is found as good it won't be
-highlighted as rare.
+a typing mistake anyway.
 
 This flag can also be used on an affix, so that a basic word is not rare but
 the basic word plus affix is rare |spell-affix-flags|.  However, if the word

src/nvim/spell.c

@@ -572,8 +572,8 @@ typedef struct langp_S {
 
 // Result values.  Lower number is accepted over higher one.
 #define SP_BANNED       -1
-#define SP_OK           0
-#define SP_RARE         1
+#define SP_RARE         0
+#define SP_OK           1
 #define SP_LOCAL        2
 #define SP_BAD          3