Added sample code of merge! method in hash.c.

https://github.com/ruby/ruby/pull/1652

  Patch by @selmertsx [fix GH-1652]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60273 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

hash.c

@@ -2530,11 +2530,13 @@ rb_hash_update_block_i(VALUE key, VALUE value, VALUE hash)
  *     h1 = { "a" => 100, "b" => 200 }
  *     h2 = { "b" => 254, "c" => 300 }
  *     h1.merge!(h2)   #=> {"a"=>100, "b"=>254, "c"=>300}
+ *     h1              #=> {"a"=>100, "b"=>254, "c"=>300}
  *
  *     h1 = { "a" => 100, "b" => 200 }
  *     h2 = { "b" => 254, "c" => 300 }
  *     h1.merge!(h2) { |key, v1, v2| v1 }
  *                     #=> {"a"=>100, "b"=>200, "c"=>300}
+ *     h1              #=> {"a"=>100, "b"=>200, "c"=>300}
  */
 
 static VALUE