Remove leading '$' from command snippets in README.md

[bnsmcx]

The documentation is straightforward and easy to follow and the 'copy' button is useful for quickly getting set up. However, while the leading $ does make it clear to the reader that snippet represents a CLI command, it also makes it an invalid command. The user has to go back and remove this character or else the command fails:

I can make the changes and link the pull request if the maintainers think this is a worthwhile tweak.