fix trac URL to GH URL

raster/r.in.mat/r.in.mat.html

@@ -99,8 +99,7 @@ save -mat4-binary filename.mat map_*
 
 Robust support for mixed-Endian importation.
 <i>(This is a work in progress, please help by reporting any failures to the
-<a href="http://trac.osgeo.org/grass/">
-GRASS bug tracking system</a></i>; you will need to login with an OSGeo Userid)
+<a href="https://github.com/OSGeo/grass/issues">GRASS bug tracking system</a></i>;
 <br>
 Add support for importing map history, category information, color map, etc.
 if they exist.