Anchor print on the line-end in robots.txt

In Zendesk #224071 it is noted that Google is warning that URLs like
https://www.gov.uk/government/publications/printing-immform-helpsheet-11
are not being indexed.

This seems to be because the pattern match (which isn't full regex
syntax) is too greedy, matching */print and */printfoo. By anchoring it
on the end of the line with $, it should only match /print

References:
    https://support.google.com/webmasters/answer/156449?hl=en&from=40360&rd=1
Merge pull request #363 from alphagov/anchor-print-in-robotstxt

Anchor print on the line-end in robots.txt