Fixed formatting of a Note in Training-Plugins.md

[milant]

Corrected a "Note" heading that should be bolded.

Proposed change(s)

Proper Markdown formatting to a Note heading was applied.

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments

[CLAassistant]

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[maryamhonari]

Hi @milant,

Thank you for catching this. Can you please sign the CLA so that I can merge this?