Use normal diagram directive in unicode range documentation

The diagram is only available in the specification draft, but it will be one
day back in the published version.

Author: liZe

Diff for: tinycss2/ast.py

@@ -384,7 +384,7 @@ def _serialize_to(self, write):
 
 
 class UnicodeRangeToken(Node):
-    """A `unicode-range token <https://www.w3.org/TR/css-syntax-3/#urange>`_.
+    """A :diagram:`unicode-range-token`.
 
     .. autoattribute:: type