Update website to reflect new brew command syntax #601

Describe the bug
The websites installation instructions states to use brew cask install qtpass, the git repo README.md already has the updated command syntax brew install --cask qtpass.

To Reproduce
Steps to reproduce the behavior:

1. Try to execute the first command on https://qtpass.org/downloads#macOS
2. See error: Error: 'brew cask' is no longer a 'brew' command. Use 'brew <command> --cask' instead.

Expected behavior
Installation via brew.

Desktop (please complete the following information):

• OS/Distribution: macOS Ventura 13.0.1 (22A400)
• Version [e.g. 1.2.3]