Fix the URL for checking out a PR to FETCH_HEAD

I'm pretty sure this example if missing `/head` on the end, because when
I do this it fails:

```
$ git fetch origin refs/pull/70
fatal: Couldn't find remote ref refs/pull/70
fatal: The remote end hung up unexpectedly
```

Whereas when I do this it works:

```
$ git fetch origin refs/pull/70/head
From github.internal:organisation/example-repo
 * branch            refs/pull/70/head -> FETCH_HEAD
```