Update PR template

***NO_CI***

Author: josesimoes

.github/PULL_REQUEST_TEMPLATE.md

@@ -18,4 +18,4 @@
 <!--- Go over all the following points, and put an `x` in all the boxes that apply. -->
 <!--- If you're unsure about any of these, don't hesitate to ask. We're here to help! -->
 - [ ] My doc follows the code style of this project.
-- [ ] I have read the [CONTRIBUTING](https://github.com/nanoframework/.github/blob/master/CONTRIBUTING.md) document.
+- [ ] I have read the [CONTRIBUTING](https://github.com/nanoframework/.github/blob/main/CONTRIBUTING.md) document.