Fix broken link in spec

specification/appendices/signing_json.rst

@@ -91,7 +91,7 @@ Signing Details
 
 JSON is signed by encoding the JSON object without ``signatures`` or keys grouped
 as ``unsigned``, using the canonical encoding described above. The JSON bytes are then signed using the
-signature algorithm and the signature is encoded using `unpadded Base64`._
+signature algorithm and the signature is encoded using `unpadded Base64`_.
 The resulting base64 signature is added to an object under the
 *signing key identifier* which is added to the ``signatures`` object under the
 name of the entity signing it which is added back to the original JSON object