use .md extension

Author: gregsdennis

specs/jsonschema-validation.md

@@ -211,7 +211,7 @@ Omitting this keyword has the same behavior as a value of 0.
 
 The value of this keyword MUST be a string. This string SHOULD be a valid
 regular expression as indicated in
-[JSON Schema Core, section 6.3](./jsonschema-core.html#regex).
+[JSON Schema Core, section 6.3](./jsonschema-core.md#regex).
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 A string instance is considered valid if the regular expression matches the