Fix fmt verb typo in helper error message

Found by `go vet`:

    ➜  cdn-acceptance-tests git:(master) ✗ go vet
    helpers.go:88: unrecognized printf verb 'n'
    exit status 1

Should be a base10 integer. Otherwise the following message is generated:

    2014/06/12 11:55:50 CDN still not available after %!n(int=20) attempts
Merge pull request #30 from alphagov/fmt_verb_typo

Fix fmt verb typo in helper error message