[Docs] Modify pytorch2onnx docs (#5269)
[Docs] Modify pytorch2onnx docs (#5269)

* modify onnx docs

* modify onnx docs

* fixgrammatical errors

* we -> We

* refer to -> following

* use a more accurate reference

* modify the expression

* modify according to new get_start.md
open-mmlab/mmdetection,open-mmlab/mmdetection