docs: update assert TypeScript declaration examples

[stdlib-bot]

This PR updates TypeScript declarations for the @stdlib/assert namespace.

Reviewer Checklist

• Check the scope of the changes (following Conventional Commits):
  • Are these new APIs? Then this is a feat.
  • Are these changes to existing APIs that could break compatibility? Then this is a feat! (i.e., a breaking change).
  • Are these only documentation changes to existing APIs? Then this is docs.
• Update the PR title to align with the change type (feat, feat!, or docs).
• Approve the PR once you are confident about the classification and changes made.

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
assert	$\color{red}45452/48917$ $\color{green}+92.92%$	$\color{red}877/1004$ $\color{green}+87.35%$	$\color{red}62/422$ $\color{green}+14.69%$	$\color{red}45452/48917$ $\color{green}+92.92%$

The above coverage report was generated for the changes in this PR.