[caffe] fix input order in SLS op documentation (#36708)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36708
WEIGHTS is the second input operand of SparseLengthsWeightedSum operators but in the documentation the order was wrong.
Test Plan: CI
Reviewed By: yinghai
Differential Revision: D21058240
fbshipit-source-id: e160e983603e606e63fbbfdee34d98d3587870d8
