fix brew install in README

[0xdevalias]

Fix the brew install command to use the modern syntax, as brew cask install is no longer supported.

Description

Motivation and Context

⇒  brew cask install franz
Error: `brew cask` is no longer a `brew` command. Use `brew <command> --cask` instead.

How Has This Been Tested?

N/A

Screenshots (if appropriate):

N/A

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] My code follows the code style of this project (run $ yarn lint).
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.

[0xdevalias]

Whoops.. seems like there are already a few duplicate PRs to fix this:

• https://github.com/meetfranz/franz/pull/2003
• https://github.com/meetfranz/franz/pull/2148

And not strictly a duplicate, but another update to the homebrew section of the docs:

• https://github.com/meetfranz/franz/pull/1660

[bircni]

I think this could be a helpful merge - i dont know why this not being approved

[hkdavid2008]

the problem is the top window always....