Update README to match required usage

[adamtaylor13]

Running the brew install command from the README yielded:

`brew extract` or `brew create` and `brew tap-new` to create a formula file in a tap on GitHub instead.: Invalid usage: Non-checksummed download of pup formula file from an arbitrary URL is unsupported!  (UsageError)
`brew extract` or `brew create` and `brew tap-new` to create a formula file in a tap on GitHub instead.: Invalid usage: Non-checksummed download of pup formula file from an arbitrary URL is unsupported!  (UsageError)

But the following command:

brew install pup

Worked perfectly.

[gromgit]

There are already at least two open PRs for this, the earliest of which dates back to 2019: #116, #155.

[adamtaylor13]

@ericchiang Can we get this quick & easy README update merged?

[adamtaylor13]

@ericchiang Just pinging this again. It's a very small update to the README which will help future users install this tool more easily.