DoxygenLua: fix closing comment blocks

Summary:
Kate-editor needs the context to be marked dynamic to use a placeholder in a dynamic rule.

BUG: 394184

Test Plan:
Open Kate with the code lua below:

```
--[[
commented out
]]
local test
```

An equivalent is already in autotests/input/highlight.lua

Reviewers: #framework_syntax_highlighting, dhaumann

Reviewed By: #framework_syntax_highlighting, dhaumann

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D12860