Wrong LaTeX syntax in the SGD operator comment

[wangkuiyi]

Paddle/paddle/operators/sgd_op.cc

Lines 56 to 58 in d883547

	This operator implements one step of the stochastic gradient descent algorithm.
	$$param_out = param - learning_rate * grad$$

displays as

[abhinavarora]

@wangkuiyi I think we will encounter the same problem in many other ops. Do you know where I can see the generated documents?

[luotao1]

@abhinavarora You can see the generated documents in http://www.paddlepaddle.org/docs/develop/documentation/zh/operators.html