Rank loss op doc displayed incorrectly on paddle.org

[kexinzhao]

Paddle/paddle/operators/rank_loss_op.cc

Lines 71 to 75 in 3557235

	\f$$
	C_{i,j} = -\tilde{P_{ij}} * o_{i,j} + log(1 + e^{o_{i,j}}) \\
	o_{i,j} = o_i - o_j \\
	\tilde{P_{i,j}} = \left \{0, 0.5, 1 \right \} \ or \ \left \{0, 1 \right \}
	\f$$

\f should be removed.

Correctly, it is incorrectly displayed here:
http://www.paddlepaddle.org/docs/develop/documentation/en/operators.html#permalink-82-rank_loss

[kuke]

Thanks. I will fix it