fix the diff button location

[ikiwiki] / doc / pagehistory.mdwn

ikiwiki supports adding "History" links to the top of pages to browse the
revision history of a page. This is enabled by the `historyurl` setting,
which is used to specify the URL to a web interface such as [[ViewVC]]
(for Subversion) or [[Gitweb]]. In that url, "\[[file]]" is replaced with
the name of the file to view.

The [[plugins/repolist]] plugin can supplement this information with
urls to the underlying repository of the wiki.