在给定的文本中找到并替换指定单词，替换为指定的单词。直接输出替换后的文本。
原始文本：Five little monkeys jumping on the bed. One fell off and bumped his head.
要替换的单词：monkeys
替换的单词：kittens
Five little kittens jumping on the bed. One fell off and bumped his head.