libtiff URL update (trac https://trac.osgeo.org/grass/ticket/768)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@39348 15284696-431f-4ddb-bdfa-cd5b030d7da7

REQUIREMENTS.html

@@ -116,7 +116,7 @@ use it at time!</I>
 
 
 <li><B>libtiff</B> (for r.out.tiff), usually already installed.<br>
-<a href="http://www.libtiff.org">http://www.libtiff.org</a>
+<a href="http://www.remotesensing.org/libtiff/">http://www.remotesensing.org/libtiff/</a>
 
 <li><B>readline</B> for extra command prompt functionality
 <br><a href="http://tiswww.case.edu/~chet/readline/rltop.html">http://tiswww.case.edu/~chet/readline/rltop.html</a>