Linux brew command is wrong in documentation

## Environment

* Operating system (including version): UBUNTU
* mkcert version (from `mkcert -version`):
* Server (where the certificate is loaded):
* Client (e.g. browser, CLI tool, or script):

## What you did

I was intsalling mckert on ubuntu and used the command brew install mkcert
## What went wrong

aashi@Syzama:~/glific$ brew install mkcert
Command 'brew' not found, did you mean:
command 'brec' from deb bplay (0.991-10build1)
command 'qbrew' from deb qbrew (0.4.1-8build1)
Try: sudo apt install

need to change this in documentation as this command does not work in ubuntu. instead The brew command is specific to macOS and is not available on Linux systems. In the case of Ubuntu or other Debian-based distributions, you can use the apt package manager to install mkcert. Here's the command you can use:
`sudo apt update
sudo apt install mkcert
`