Clarify argument to dict macro

d is more clear because the type of argument is dict not dictht

Closes #513

src/dict.h

@@ -142,7 +142,7 @@ typedef void (dictScanFunction)(void *privdata, const dictEntry *de);
 #define dictGetDoubleVal(he) ((he)->v.d)
 #define dictSlots(d) ((d)->ht[0].size+(d)->ht[1].size)
 #define dictSize(d) ((d)->ht[0].used+(d)->ht[1].used)
-#define dictIsRehashing(ht) ((ht)->rehashidx != -1)
+#define dictIsRehashing(d) ((d)->rehashidx != -1)
 
 /* API */
 dict *dictCreate(dictType *type, void *privDataPtr);