Fix OBJECT arity

Previously, the command definition for the OBJECT command specified
a minimum of two args (and that it was variadic), which meant that
if you sent this:

OBJECT foo

When cluster was enabled, it would result in an assertion/SEGFAULT
when Redis was attempting to extract keys.

It appears that OBJECT is not variadic, and only ever takes 3 args.

https://gist.github.com/michael-grunder/25960ce1508396d0d36a

src/object.c

@@ -696,7 +696,7 @@ robj *objectCommandLookupOrReply(redisClient *c, robj *key, robj *reply) {
 }
 
 /* Object command allows to inspect the internals of an Redis Object.
- * Usage: OBJECT <verb> ... arguments ... */
+ * Usage: OBJECT <refcount|encoding|idletime> <key> */
 void objectCommand(redisClient *c) {
     robj *o;