[ticket/12406] Fix variable description of core.viewtopic_modify_page_title

Variable page_title should be described as "Title of the viewtopic page".

PHPBB3-12406
Merge pull request #2325 from n-aleha/ticket/12406

[ticket/12406] Fix variable description of core.viewtopic_modify_page_ti...

* n-aleha/ticket/12406:
  [ticket/12406] Fix variable description of core.viewtopic_modify_page_title