[ticket/12391] Assign variables to core.posting_modify_template_vars

Assign variables to event "core.posting_modify_template_vars".
Also, fix a couple of typos.

PHPBB3-12391
Merge pull request #2305 from n-aleha/ticket/12391

[ticket/12391] Assign variables to core.posting_modify_template_vars

* n-aleha/ticket/12391:
  [ticket/12391] Assign variables to core.posting_modify_template_vars