change clone command

[oxarbitrage]

When i try to clone with the provided command in the README i get the error:

oxarbitrage@oxarbitrage-Lenovo-ideapad-320S-14IKB:~$ git clone git@github.com:bitshares/beet.git
Cloning into 'beet'...
Warning: Permanently added the RSA host key for IP address '140.82.114.4' to the list of known hosts.
git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

This pull changes the git command and fixes the issue.