minor Fix deprecations: curly string access

[mdeweerd]

Fix use of access to string using curly brackets https://www.php.net/manual/en/migration74.deprecated.php

Replaces with square brackets.

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌
Tests pass?	❌
Fixed issues

[what-the-diff]

PR Summary

• Update to Markdown Parsing Logic in MarkdownParser.php
  The developer updated a particular line of code related to header level information parsing in the Markdown Parser. This may lead to improved handling of headers in our markdown files.

• Same Change Implemented in Viewer Component
  The same adjustment was made in the Markdown Parser within the 'viewer/components' directory. It ensures uniformity and consistency in parsing markdown files across other components in our system.