Fix Homebrew command

[carstenjacobsen]

What did you do?

The command for installing the CLI with Homebrew in the docs no longer works.

What did you expect to see?

Installation of the Stellar/Soroban CLI was expected.

What did you see instead?

This error message is thrown: No available formula or cask with the name "stellar/tap/stellar-cli". Did you mean stellar/tap/stellar-xdr or stellar/tap/soroban-cli?

If soroban-cli is installed using Homebrew, all the command line-based instructions in the Getting Started guide won't work, because only support for using soroban commands will be supported (e.g. soroban contract build).

When installing the CLI with cargo, stellar and soroban can be used interchangeably in command line. This is not the case when installing the CLI with Homebrew, so consider changing the commands in the Getting Started guide to using soroban.