documentation: Updated git checkout command and reorder unit test steps

[ca-nguyen]

Issue #, if available:

Description of changes:

• Updated git checkout command to checkout main (and not master).
• Reordered unit test steps: pip install must be done inside the project folder.

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[StepFunctions-Bot]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildProject6AEA49D1-sEHrOdk7acJc
• Commit ID: ad112f802693f2bf2b1e1c48fdc4df00faa5081d
• Result: SUCCEEDED
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