Update command about Homebrew cask option

[ktanoooo]

Description

Update some markdown files about Homebrew cask option.
At latest homebrew version, cask option was changed from "brew cask install **" to "brew install --cask **"
In this PR(#552), only readme.md has changed .

Requirements / Checklist

• Read the Contributing Guidelines
• Read or at least glanced at the FAQ
• Read or at least glanced at the Wiki
• Scripts execute without error (if necessary):
  • If any of the scripts were modified they have been tested and execute without error, e.g.:
    • ./font-patcher Inconsolata.otf --fontawesome --octicons --pomicons
    • ./gotta-patch-em-all-font-patcher\!.sh Hermit
• Extended the README and documentation if necessary, e.g. You added a new font please update the table

What does this Pull Request (PR) do?

Update some markdown files.

How should this be manually tested?

Any background context you can provide?

What are the relevant tickets (if any)?

#552

Screenshots (if appropriate or helpful)

[Finii]

This looks great! Someone knows git grep ;-)
Thanks a lot.