Fix remaining x-common references

docs/MAINTAINING.md

 
 ### Exercism-wide
 
-- [x-common](https://github.com/exercism/x-common) — the library of canonical exercises.
+- [problem-specifications](https://github.com/exercism/problem-specifications) — the library of canonical exercises.
 - [discussions](https://github.com/exercism/discussions) — the place where project-wide conversations happen. 
   [issues sorted by most recently updated.](https://github.com/exercism/discussions/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc)
 - [docs](https://github.com/exercism/docs) - the place to find project-wide documentation.

exercises/hello-world/TUTORIAL.md

 
 A starting point to jumping in can be found here:
 
-https://github.com/exercism/x-common/blob/master/CONTRIBUTING.md
+https://github.com/exercism/problem-specifications/blob/master/CONTRIBUTING.md
 
 
 ----

exercises/perfect-numbers/src/test/java/NaturalNumberTest.java

 
 /*
  * version: 1.0.0
- * https://github.com/exercism/x-common/blob/19d0c7714ce664a1ad762af624c17f8e269fa8b2/exercises/perfect-numbers/canonical-data.json
  */
 public class NaturalNumberTest {