doc: fix PR template link to CONTRIBUTING (#1347)

The current PR template links to the backstage/backstage CONTRIBUTING.md
document however we have one in this repo which is tailored to the repo, so this
change updates the link.

Signed-off-by: Crevil <bjoern.soerensen@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -7,8 +7,8 @@
 
 <!--- Please include the following in your Pull Request when applicable: -->
 
-- [ ] A changeset describing the change and affected packages. ([more info](https://github.com/backstage/backstage/blob/master/CONTRIBUTING.md#creating-changesets))
+- [ ] A changeset describing the change and affected packages. ([more info](https://github.com/backstage/community-plugins/blob/master/CONTRIBUTING.md#creating-changesets))
 - [ ] Added or updated documentation
 - [ ] Tests for new functionality and regression tests for bug fixes
 - [ ] Screenshots attached (for UI changes)
-- [ ] All your commits have a `Signed-off-by` line in the message. ([more info](https://github.com/backstage/backstage/blob/master/CONTRIBUTING.md#developer-certificate-of-origin))
+- [ ] All your commits have a `Signed-off-by` line in the message. ([more info](https://github.com/backstage/community-plugins/blob/master/CONTRIBUTING.md#developer-certificate-of-origin))