[x/tour] content: add a link to previous exercise in methods/25
[x/tour] content: add a link to previous exercise in methods/25

Added a link to methods/25 to moretypes/18 because they are related.
This improves the usability of the page.

Fixes golang/tour#95

Change-Id: I3ec7e85267d0e5da9c67194dce58d5310eefc97b
GitHub-Last-Rev: 1fe64640097e65af0ddbd058b908f765a9aaf7da
GitHub-Pull-Request: golang/tour#471
Reviewed-on: https://go-review.googlesource.com/110118
Reviewed-by: Alberto Donizetti <90b16103a35511ced020676f3544f17117fb4c92@gmail.com>
X-Tour-Commit: 03133cc9684f0d84f4b533c0b064683d8822e587

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website