[#4808] s/Forge/Allura/g in README.markdown
[#4808] s/Forge/Allura/g in README.markdown

Way back before Allura was named, 'Forge' was used.  Better to just call
things Allura now.

apache/allura,apache/allura,heiths/allura,apache/incubator-allura,apache/allura,apache/incubator-allura,apache/incubator-allura,lym/allura-git,lym/allura-git,lym/allura-git,apache/allura,apache/incubator-allura,heiths/allura,heiths/allura,heiths/allura,apache/allura,heiths/allura,lym/allura-git,lym/allura-git