update migrating.md

[hamidrexa]

Proposed change(s)

Fix markdown indentation

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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[alex-mccarthy-unity]

Thanks for the improvement! I'll merge this in.