chore: use more explicit relative URL for PR template

Addresses #42

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,4 @@
-<!--- IMPORTANT: Please review [how to contribute](CONTRIBUTING.md) before proceeding further. -->
+<!--- IMPORTANT: Please review [how to contribute](/immuni-app/immuni-app-ios/blob/master/CONTRIBUTING.md) before proceeding further. -->
 <!--- IMPORTANT: If this is a Work in Progress PR, please mark it as such in GitHub. -->
 
 ## Description
@@ -17,7 +17,7 @@ In particular, the ...
 
 <!--- Please insert an ‘x’ after you complete each step -->
 
-- [ ] I have followed the indications in the [CONTRIBUTING](CONTRIBUTING.md) document.
+- [ ] I have followed the indications in the [CONTRIBUTING](/immuni-app/immuni-app-ios/blob/master/CONTRIBUTING.md) document.
 - [ ] The documentation related to the proposed change has been updated accordingly (plus comments in code).
 - [ ] I have written new tests for my core changes, as applicable.
 - [ ] I have successfully run tests with my changes locally.