Revert "Merge pull request #15446 from akshay-vishnoi/doc_changes"

It was right as originally written in #15440.

activesupport/lib/active_support/hash_with_indifferent_access.rb

@@ -221,7 +221,7 @@ def values_at(*indices)
     #   dup  = hash.dup
     #   dup[:a][:c] = 'c'
     #
-    #   hash[:a][:c] # => nil
+    #   hash[:a][:c] # => "c"
     #   dup[:a][:c]  # => "c"
     def dup
       self.class.new(self).tap do |new_hash|