[DOC] Fix call-seq for GC methods

RDoc parses the last arrow in the call-seq as the arrow for the return
type. It was getting confused over the arrow in the hash.

gc.rb

@@ -219,7 +219,7 @@ module GC
   end
 
   #  call-seq:
-  #     GC.latest_gc_info -> {:gc_by=>:newobj}
+  #     GC.latest_gc_info -> hash
   #     GC.latest_gc_info(hash) -> hash
   #     GC.latest_gc_info(:major_by) -> :malloc
   #