link to 2.7 generated references, not latest

When viewing the Puppet 2.7 docs, links to Generated References
should link to 2.7.latest to align with the version they're looking at.
Before this commit it linked to 'latest', or version 3 which may cause
confusion.
Merge pull request #225 from 3flex/2.7-is-not-latest

(#21215) link to 2.7 generated references, not latest