Escape `\$` in PS1

The `\$` in PS1 needed to be escaped so that it would work as intended. (switch to `#` if you were root)

Minor change, but I noticed it after running `sudo -i` (I thought my sudo had failed for a split second).

I believe bash is evaluating the `\$` at runtime/string evaluation to mean "I don't actually mean this as a variable", and it's never making it to the PS1 env var.