[doc] groupNormalizationPlugin loss readme.md

[lix19937]

From trt 7.1 to latest version, this sample plugin loss readme.md.

https://github.com/NVIDIA/TensorRT/tree/release/7.1/plugin/groupNormalizationPlugin

[poweiw]

@lix19937 Thanks for the report! An internal bug has been filed and should be fixed in the next few releases