Fix issue links

There's no real need to have the full URL inside the README. Should the name of the repo change for some reason (or the URL to issues), just writing #<number of issue/PR> will still work. These two issue links didn't work either way (it's `/issues` not `/issue`).

Since now the raw README format is inconsistent I wanted to ask you if I should change all links to the short syntax or just add the `s` to the links and continue to use your style.