Make the link easier to copy. (#28516)

Summary:
Making a PR from GitHub, I need to copy-paste the link, and it would be easier to just triple-click a line with the URL rather than carefully selecting the URL from the text.

<img width="723" alt="Screen Shot 2020-04-03 at 17 33 47" src="https://user-images.githubusercontent.com/100233/78378550-6c12af80-75d1-11ea-93a4-2eae568ce602.png">

## Changelog

[General] [Changed] - Make PR template easier to use with changelog URL.
Pull Request resolved: https://github.com/facebook/react-native/pull/28516

Reviewed By: fkgozali

Differential Revision: D20842238

Pulled By: hramos

fbshipit-source-id: 3fef7a994f36a996bbbc52556600d468a56210a9