Fix incorrect measures link URL
Fix incorrect measures link URL

The previous version omitted the trailing slash, which works but causes
a needless redirect. But we should be using the `url` tag anyway, rather
than constructing URLs by hand.

ebmdatalab/openprescribing,annapowellsmith/openpresc,annapowellsmith/openpresc,annapowellsmith/openpresc,ebmdatalab/openprescribing,annapowellsmith/openpresc,ebmdatalab/openprescribing,ebmdatalab/openprescribing