docs: Fix $indexPage param explanations in helpers/html_helper.rst

[obozdag]

Description Fix $indexPage parameter explanations in helpers/html_helper.rst

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPDoc blocks, only if necessary or adds value
• [ ] Unit testing, with >80% coverage
• [x] User guide updated
• [ ] Conforms to style guide