doc: add missing install subcommand for brew command to install the plugin

[KeisukeYamashita]

Description

brew install <PLUGIN> is the correct command. Without install it will run into an error:

Error: Unknown command: brew kafkactl-azure-plugin

Fixes # None

Type of change

Please delete options that are not relevant.

• 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)

Documentation

• the change is mentioned in the ## [Unreleased] section of <plugin>/CHANGELOG.md
• the configuration options in <plugin>/README.adoc were updated