Fix snapshot docs URL, currently only appears on snapshot docs

[jamesfredley]

Fixes the snapshot URL in the dropdown on https://docs.grails.org/snapshot/guide/single.html

https://docs.grails.org/7.0.0-SNAPSHOT/guide/single.html should be https://docs.grails.org/snapshot/guide/single.html

<select onchange="window.document.location.href=this.options[this.selectedIndex].value;"> <option selected="selected" value="https://docs.grails.org/snapshot/guide/single.html">SNAPSHOT</option> <option selected="selected" value="https://docs.grails.org/7.0.0-SNAPSHOT/guide/single.html">7.0.0-SNAPSHOT</option></select> 

with

<select onchange="window.document.location.href=this.options[this.selectedIndex].value;"> <option selected="selected" value="https://docs.grails.org/snapshot/guide/single.html">SNAPSHOT</option> <option selected="selected" value="https://docs.grails.org/snapshot/guide/single.html">7.0.0-SNAPSHOT</option> </select>