message: [docs] Update link to relative path
diff: FX2TRT is merged as FX module in Torch-TensorRT
  -- The user guide is in [link](https://github.com/pytorch/TensorRT/blob/master/docsrc/tutorials/getting_started_with_fx_path.rst#installation)
  -- The examples are moved to [link](https://github.com/pytorch/TensorRT/tree/master/examples/fx_example)
  +- The user guide is in [link](../../../docsrc/tutorials/getting_started_with_fx_path.rst#installation)
  +- The examples are moved to [link](../../../examples/fx)
  