LSTM op doc not displayed correctly on paddle.org

[kexinzhao]

https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/operators/lstm_op.cc#L201-L222

I think all the /f can be removed from this code.

Currently /f leading to incorrect display on:
http://www.paddlepaddle.org/docs/develop/documentation/en/operators.html#permalink-12-lstm

[qingqing01]

@kexinzhao Thanks! I'll fix it. :)