bpo-44320: Fix markup for W3C C14N test suite (GH-26556)

Author: osamu0329nakamura

Doc/license.rst

@@ -955,7 +955,7 @@ W3C C14N test suite
 The C14N 2.0 test suite in the :mod:`test` package
 (``Lib/test/xmltestdata/c14n-20/``) was retrieved from the W3C website at
 https://www.w3.org/TR/xml-c14n2-testcases/ and is distributed under the
-3-clause BSD license:
+3-clause BSD license::
 
    Copyright (c) 2013 W3C(R) (MIT, ERCIM, Keio, Beihang),
    All Rights Reserved.