Update OP Lowering guide

Author: JackCaoG

OP_LOWERING_GUIDE.md

@@ -24,4 +24,4 @@ All file mentioned below lives under the `xla/torch_xla/csrc` folder, with the e
 Our CircleCI runs PyTorch native python tests for every change and every day. Those tests will use XLA implementation if we provide a lowering. We usually don’t need to add additional python tests for PyTorch/XLA unless we want to verify some xla behaviors(like dynamic shape) or we skipped the pytorch native test for some reason. The python test should be added to `xla/test/test_operations.py` if it is required. We also need to add CPP tests in `xla/test/cpp/test_aten_xla_tensor.cpp`. This test should call PyTorch c++ API and verify our implementation yields the same result as PyTorch native implementation. We also need to verify if the xla implementation is called when the tensor is a XLA tensor by checking the `aten::op` and `xla::op` counters.
 
 ## Tips
-The process of lowering is breaking down the PyTorch operations into a sequence of XlaOp. To provide a good lowering of the PyTorch operation, one needs to have a good grasp of what XLA is capable of. Reading the XlaOp document and looking into how similar ops is lowered is the best way to achieve that. You can find a minimal Op lowering example in [this pr](https://github.com/pytorch/xla/pull/2969). You can also find a slightly more complicated example with backward lowering in [this pr](https://github.com/pytorch/xla/pull/1940).
+The process of lowering is breaking down the PyTorch operations into a sequence of XlaOp. To provide a good lowering of the PyTorch operation, one needs to have a good grasp of what XLA is capable of. Reading the XlaOp document and looking into how similar ops is lowered is the best way to achieve that. You can find a minimal Op lowering example in [this pr](https://github.com/pytorch/xla/pull/2969). You can also find a slightly more complicated example with backward lowering in [this pr](https://github.com/pytorch/xla/pull/2972).