lavfi/xbr: update filter url

Existing link is broken.
This patch updates the existing url with a working one.
Signed-off-by: Mina <minasamy_@hotmail.com>
Signed-off-by: Gyan Doshi <ffmpeg@gyani.pro>

doc/filters.texi

@@ -17148,7 +17148,7 @@ separatefields,select=eq(mod(n,4),0)+eq(mod(n,4),3),weave
 @section xbr
 Apply the xBR high-quality magnification filter which is designed for pixel
 art. It follows a set of edge-detection rules, see
-@url{http://www.libretro.com/forums/viewtopic.php?f=6&t=134}.
+@url{https://forums.libretro.com/t/xbr-algorithm-tutorial/123}.
 
 It accepts the following option:
 

libavfilter/vf_xbr.c

@@ -24,7 +24,7 @@
  * XBR Filter is used for depixelization of image.
  * This is based on Hyllian's xBR shader.
  *
- * @see http://www.libretro.com/forums/viewtopic.php?f=6&t=134
+ * @see https://forums.libretro.com/t/xbr-algorithm-tutorial/123
  * @see https://github.com/yoyofr/iFBA/blob/master/fba_src/src/intf/video/scalers/xbr.cpp
  */