Fix bug to allow highlighting of code with '-'

[yucheng11122017]

Note: this PR is a follow-up to #2125 to rectify the accidental merge.

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Overview of changes:
Trying to highlight code which contained a '-' did not work since the highlight rules were parsed according to '-'.
Eg. {highlight-lines="1['block-level']"}
Change to parse using regex in which '-' cannot be within a [].

Anything you'd like to highlight/discuss:

Testing instructions:

 ```{highlight-lines="1['block-level']"}
<markdown> elements allow block-level markdown without needing a leading newline.
Hence, the contained markdown should be parsed and output as is, without any parsing errors.
</invalidhtml>
 ```

Proposed commit message: (wrap lines at 72 characters)
Fix bug to allow highlighting of code with '-'

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

[ong6]

LGTM

[jovyntls]

Working on my end! LGTM

[tlylt]

@all-contributors please add @yucheng11122017 for code

[allcontributors]

@tlylt

I've put up a pull request to add @yucheng11122017! 🎉