use standard link syntax for cross-file references

Author: gregsdennis

specs/jsonschema-validation.md

@@ -208,8 +208,7 @@ Omitting this keyword has the same behavior as a value of 0.
 #### `pattern` {#pattern}
 
 The value of this keyword MUST be a string. This string SHOULD be a valid
-regular expression as indicated in {{jsonschema-core#regex}}.
-<!-- need to verify the {{section}} syntax works across files somehow -->
+regular expression as indicated in [JSON Schema Core, section 6.3](./jsonschema-core.md#regex).
 
 A string instance is considered valid if the regular expression matches the
 instance successfully. Recall: regular expressions are not implicitly anchored.