Update CONTRIBUTING.md (#67)

motivation: move from `master` to `main`, also make link less fragile

changes: make link relative

Author: tomerd

CONTRIBUTING.md

@@ -70,7 +70,7 @@ For this reason, whenever you add new tests **you have to run a script** that ge
 
 ### Run `./scripts/sanity.sh`
 
-The scripts directory contains a [sanity.sh script](https://github.com/swift-server/swift-service-lifecycle/blob/master/scripts/sanity.sh) 
+The scripts directory contains a [sanity.sh script](scripts/sanity.sh) 
 that enforces additional checks, like license headers and formatting style.
 Please make sure to `./scripts/sanity.sh` before pushing a change upstream, otherwise it is likely the PR validation will fail
 on minor changes such as a missing `self.` or similar formatting issues.