fix: Update conda instructions in Developer Setup.md

[BrendanWalsh]

conda create doesn't automatically activate the conda env, so added an instruction to explicitly activate the environment with conda activate

[acrolinxatmsft1]

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A minimum Acrolinx score of 80 is required.

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Article	Acrolinx score	Word and Phrases Score	Correctness Score	Scorecard	Processed
docs/Reference/Developer Setup.md	87	100	60	link	✅

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[github-actions]

Hey @BrendanWalsh 👋!
Thank you so much for contributing to our repository 🙌.
Someone from SynapseML Team will be reviewing this pull request soon.

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