update documentation in OpenSim.ini.example for j2k decode cache expiration time

bin/OpenSim.ini.example

@@ -1375,6 +1375,8 @@
 
 [J2KDecoder]
 ;CacheDir = "./j2kDecodeCache"
+; Time in minutes before cached decodes expire. Set to 0 for no expiration. Default is 720 minutes.
+
 ;CacheTimeout = 720
 
 ;;