Fix link in patch update guide

Use an absolute URL to gitlab.com's master branch instead of a relative URL on the local instance.  Also change which words are include in the anchor element, since the Markdown parser doesn't seem to like backticks inside the link text.

### Before

![update_link_before](https://dev.gitlab.org/Okada/gitlabhq/uploads/f6a8b2aa06ca210d5ea8a8b28a78c30c/update_link_before.png)

### After

![update_link_after](https://dev.gitlab.org/Okada/gitlabhq/uploads/6df92b3f5d8ffd6906bdc62cf19acb11/update_link_after.png)

cc @sytse

See merge request !1735