docs: Converted PR template brackets to checkboxes

Author: edcarroll

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 Before submitting a pull request, please make sure you have at least performed the following:
 
- - [] read and followed the [CONTRIBUTING.md](https://github.com/edcarroll/ng2-semantic-ui/blob/master/CONTRIBUTING.md#) guide.
- - [] linted, built and tested the changes locally.
- - [] added/updated any applicable API documentation.
- - [] added/updated any applicable demos.
+ - [ ] read and followed the [CONTRIBUTING.md](https://github.com/edcarroll/ng2-semantic-ui/blob/master/CONTRIBUTING.md#) guide.
+ - [ ] linted, built and tested the changes locally.
+ - [ ] added/updated any applicable API documentation.
+ - [ ] added/updated any applicable demos.