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

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

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