config.txt: update documentation URL

stage1/00-boot-files/files/config.txt

@@ -1,5 +1,5 @@
 # For more options and information see
-# http://www.raspberrypi.org/documentation/configuration/config-txt.md
+# http://rpf.io/configtxtreadme
 # Some settings may impact device functionality. See link above for details
 
 # uncomment if you get no picture on HDMI for a default "safe" mode