doc: fix link to the ISC license text in the developer documentation.

doc/developer.texi

@@ -286,7 +286,7 @@ Contributions should be licensed under the
 @uref{http://www.gnu.org/licenses/lgpl-2.1.html, LGPL 2.1},
 including an "or any later version" clause, or, if you prefer
 a gift-style license, the
-@uref{http://www.isc.org/software/license/, ISC} or
+@uref{http://opensource.org/licenses/isc-license.txt, ISC} or
 @uref{http://mit-license.org/, MIT} license.
 @uref{http://www.gnu.org/licenses/gpl-2.0.html, GPL 2} including
 an "or any later version" clause is also acceptable, but LGPL is