Remove line about NETWORK wildcards for Firefox

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).