LaTeX: fix math parentheses in optional labels

Summary:
BUG: 418979

Single parentheses within [...] were highlighted as math text, but the math text is \(...\).

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann, jpoelen

Reviewed By: Framework: Syntax Highlighting, cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D28303