fix wikitoolbar help url when redmine on subpath.

if `Redmine::Utils.relative_url_root` return subpath, wikitoolbar_for method return wrong URL.

ex. relative_url_root == '/redmine', wikitoolbar_for() return '/redmine/redmine/plugin_assets/redmine_restructuredtext_formatter/help/restructuredtext_syntax.html'.

`Engines::RailsExtensions::AssetHelpers.plugin_asset_path` already include `relative_url_root` at 
https://github.com/redmine/redmine/commit/ebb445c36468529cc7873136469a6162660ad253 .