chore(readme): update install code fences

[greyscaled]

👋🏻 Hey friends, I enjoy using this project, and thought I'd push up a slightly opinionated take on the install scripts in the Readme.

The story:

I often go to the readme and press the copy to clipboard button that GitHub renders:

and then paste it into my terminal. However, this adds a preceeding $, which causes the command to fail:

$ $ go install github.com/pressly/goose/v3/cmd/goose@latest
$: command not found

The change:

The change adds a code fence with the shell language, which (at least on my browser and os) looks idential in the GH UI, but when I copy the command to the clipboard, I can use it without modification.

To see what it looks like, here's the link to the install section in my fork: https:/greyscaled/goose#install

and a side-by-side (original left, my fork right):

LMK what you think, thanks!

[mfridman]

Makes perfect sense to me. Thanks for giving goose a try and contributing 🎉