Fix typos in reduce_mean's doc
Fix typos in reduce_mean's doc

PaddlePaddle/Paddle,luotao1/Paddle,chengduoZH/Paddle,reyoung/Paddle,PaddlePaddle/Paddle,QiJune/Paddle,QiJune/Paddle,QiJune/Paddle,tensor-tang/Paddle,luotao1/Paddle,luotao1/Paddle,tensor-tang/Paddle,tensor-tang/Paddle,chengduoZH/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,chengduoZH/Paddle,luotao1/Paddle,reyoung/Paddle,jacquesqiao/Paddle,baidu/Paddle,reyoung/Paddle,reyoung/Paddle,chengduoZH/Paddle,luotao1/Paddle,jacquesqiao/Paddle,QiJune/Paddle,jacquesqiao/Paddle,luotao1/Paddle,reyoung/Paddle,baidu/Paddle,baidu/Paddle,jacquesqiao/Paddle,PaddlePaddle/Paddle,chengduoZH/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,QiJune/Paddle,tensor-tang/Paddle,PaddlePaddle/Paddle,baidu/Paddle,reyoung/Paddle,baidu/Paddle,jacquesqiao/Paddle,QiJune/Paddle,jacquesqiao/Paddle,luotao1/Paddle