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2a50bc94 - Fix `CONTRIBUTING.md` instructions to install multiple Python versions (#5015)

Fix `CONTRIBUTING.md` instructions to install multiple Python versions (#5015) ## Summary I noticed the command to install multiple Python versions was wrong as it was failing cause `toolchain` is not a known command. I looked in the `ci.yml` workflow to see which command is used there and updated the instructions accordingly. ## Test Plan I just ran the command locally. :)