Fix type of Hashie::Mash (#910)

The Mash class does not inherit from stdlib's Hash but instead from
Hashie::Hash which inherits from Hash.

Author: richardboehme

gems/hashie/5.0/hashie.rbs

@@ -1,4 +1,7 @@
 module Hashie
-  class Mash < Hash[untyped, untyped]
+  class Hash < ::Hash[untyped, untyped]
+  end
+
+  class Mash < Hash
   end
 end