[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.c

@@ -10796,7 +10796,7 @@ gc_update_references(rb_objspace_t *objspace)
 #if GC_CAN_COMPILE_COMPACTION
 /*
  *  call-seq:
- *     GC.latest_compact_info -> {:considered=>{:T_CLASS=>11}, :moved=>{:T_CLASS=>11}}
+ *     GC.latest_compact_info -> hash
  *
  * Returns information about object moved in the most recent GC compaction.
  *

gc.rb

@@ -219,7 +219,7 @@ def self.stat_heap heap_name = nil, hash_or_key = nil
   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
   #