readme: use status code of `type skerrick`

[it-is-wednesday]

Minor README fix, I copied this line blindly and noticed it triggered every time I launched Emacs. Seems like my peculiar Bash installation yielded a string that's pretty far from what the line expects:

/opt/homebrew/bin/bash: line 1: type: skerrick: not found

I also tested it on an Ubuntu server and the output is different enough from the other two:

-bash: type: skerrick: not found

So I guess opting to just use the error code is preferable?

Thanks a bunch for this package<3