Add link to CHANGELOG.md

Author: ethanwharris

CHANGELOG.md

@@ -163,7 +163,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed an exception in the layer summary when the model contains torch.jit scripted submodules ([#6511](https://github.com/PyTorchLightning/pytorch-lightning/pull/6511))
 
 
-- Fixed a bug where `all_gather` would not work correctly with `tpu_cores=8`
+- Fixed a bug where `all_gather` would not work correctly with `tpu_cores=8` ([#6587](https://github.com/PyTorchLightning/pytorch-lightning/pull/6587))
 
 
 ## [1.2.3] - 2021-03-09