system/asciidoctor: fix RubyGems URL; old one is dead

system/asciidoctor/APKBUILD

@@ -9,7 +9,7 @@ arch="noarch"
 options="!check"  # can't test in this packaging format; package differently
 license="MIT"
 depends="ruby"
-source="http://gems.rubyforge.org/gems/$pkgname-$pkgver.gem"
+source="https://rubygems.org/downloads/$pkgname-$pkgver.gem"
 
 package() {
 	local gemdir="$(ruby -e 'puts Gem.default_dir')"