doc, README.md: update bug tracker URL

Closes #43

doc/mpd.1

@@ -48,7 +48,7 @@ mpd.conf(5), mpc(1)
 .SH BUGS
 If you find a bug, please report it at
 .br
-<\fBhttp://bugs.musicpd.org/bug_report_page.php\fP>.
+<\fBhttps://github.com/MusicPlayerDaemon/MPD/issues/\fP>.
 .SH AUTHORS
 Max Kellermann <max.kellermann@gmail.com>
 

doc/user.xml

@@ -1747,7 +1747,7 @@ buffer_size: 16384</programlisting>
       <para>
         If you believe you found a bug in
         <application>MPD</application>, report it on <ulink
-        url="https://bugs.musicpd.org/my_view_page.php">the bug
+        url="https://github.com/MusicPlayerDaemon/MPD/issues/">the bug
         tracker</ulink>.
       </para>