Correct access modifier

[jimrogerz]

Proposed change(s)

Correcting the access modifier for SideChannel.OnMessageReceived.

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

https://docs.unity3d.com/Packages/[email protected]/api/Unity.MLAgents.SideChannels.SideChannel.html

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• [x ] 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

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[chriselion]

@jimrogerz can you sign the CLA so that we can merge this?

[chriselion]

From https://forum.unity.com/threads/help-with-sidechannel.959023/#post-6261511

[chriselion]

I rolled this change into #4466, which also fixes the python example code.