absolute -> relative link for problem.yaml example (Kattis#10)

I think this fixes the link (absolute to relative).

Author: ghamerly

spec/problem_package_format.md

@@ -644,6 +644,6 @@ them once.
 ## See also
 
   - [Output validator](output_validators "wikilink")
-  - [Sample problem.yaml](/examples/problem_yaml "wikilink")
+  - [Sample problem.yaml](../examples/problem_yaml "wikilink")
   - [Problem format directory structure](problem_format_directory_structure "wikilink") (TBD)
   - [Problem Format Verification](problem_format_verification "wikilink") (TBD)