docs(contributing): use absolute GitHub URL for source pointer

Author: lesnik512

CONTRIBUTING.md

@@ -3,4 +3,4 @@
 The contributing guide is published as part of the project documentation:
 **https://httpware.modern-python.org/dev/contributing/**
 
-Source: [`docs/dev/contributing.md`](docs/dev/contributing.md).
+Source: [`docs/dev/contributing.md`](https://github.com/modern-python/httpware/blob/main/docs/dev/contributing.md).