docs: Remove lerna bootstrap command from setup steps

[coren-frankel]

While following the CONTRIBUTING.md setup instructions, I ran into a warning upon running the yarn bootstrap command after yarn install:

$ lerna bootstrap
ERR! bootstrap The "bootstrap" command was removed by default in v7, and is no longer maintained.
ERR! bootstrap Learn more about this change at https://lerna.js.org/docs/legacy-package-management
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

The Lerna docs say:

Replace lerna bootstrap with npm install (or yarn/pnpm). If you are already performing your package manager's install command somewhere in your workflow before where you had previously called lerna bootstrap, then you can just delete it instead.

I removed the yarn bootstrap command from the setup instructions and the bootstrap script from package.json. I also corrected some typos and grammar and made punctuation more consistent.

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