You are looking at an Eclipse p2 update site, not a web page. It is meant to be opened by Eclipse rather than by a browser, so copy the address below into Eclipse instead of browsing it here.
https://philipp0205.github.io/eclipse-pull-request-plugin/p2/
EGit must already be installed, because this site contains only the Pull Request Review plug-in and not the Eclipse platform itself.
Eclipse caches update-site metadata, including failures. Remove the entry under Preferences → Install/Update → Available Software Sites, then add the address again rather than editing the old entry.