[Installation] Change the brew cask install fastlane in any output to brew install fastlane

[SConaway]

Fastlane was migrated to homebrew-core by Homebrew/homebrew-core/47709 in December 2019, so I think that the generated README.md / output should reflect that

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

Description

Testing Steps

[googlebot]

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[SConaway]

@googlebot I already had signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[joshdholtz]

Yesssssss... thanks for making this change!

[fastlane-bot]

Hey @SConaway 👋

Thank you for your contribution to fastlane and congrats on getting this pull request merged 🎉
The code change now lives in the master branch, however it wasn't released to RubyGems yet.
We usually ship about once a week, and your PR will be included in the next one.

Please let us know if this change requires an immediate release by adding a comment here 👍
We'll notify you once we shipped a new release with your changes 🚀

[fastlane-bot]

Congratulations! 🎉 This was released as part of fastlane 2.151.0 🚀