Remove line about NETWORK wildcards for Firefox

[stuartpb]

http://codebits.glennjones.net/appcache/network/index.html indicates that Firefox has no problem with "" as a NETWORK entry, and furthermore that browsers do have problems when using "http://" and "https://*" as entries (this is the behavior I have been seeing in Chrome, and http://manifest-validator.com/validate throws up errors on those lines).