fix glog.html link to gflags repository

Fixes Issue https://github.com/google/glog/issues/325

doc/glog.html

@@ -97,7 +97,7 @@ in addition to log files.
 <h2><A NAME=flags>Setting Flags</A></h2>
 
 <p>Several flags influence glog's output behavior.
-If the <a href="http://code.google.com/p/google-gflags/">Google
+If the <a href="https://github.com/gflags/gflags">Google
 gflags library</a> is installed on your machine, the
 <code>configure</code> script (see the INSTALL file in the package for
 detail of this script) will automatically detect and use it,