Wrong LaTeX format in operator documentation

[wangkuiyi]

For an example, in lrn_op.cc we have

Paddle/paddle/operators/lrn_op.cc

Lines 91 to 96 in 32b10d3

	$$
	Output(i, x, y) = Input(i, x, y) / \left(
	k + \alpha \sum\limits^{\min(C, c + n/2)}_{j = \max(0, c - n/2)}
	(Input(j, x, y))^2
	\right)^{\beta}
	$$

This violates the rule of LaTex in Markdown -- math equations must follow the starting $$, but not on the second line. This violation leads to wrong rendering of the equation on paddlepaddle.org:

This issue is the same as https://github.com/PaddlePaddle/PaddlePaddle.org/issues/339

[shanyi15]

This problem has been repaired, please see http://www.paddlepaddle.org/docs/develop/documentation/zh/operators.html#permalink-131-lrn