Build.sh - Add cython3 requirement, fix typo in "exit"

[bartleskeet]

The build.sh file failed when running the update command on my machine, as I didn't have cython3 installed. Where it's already calling "apt-get", I think it'd make sense to add this to the packages that are installed. Also brought up here: https://github.com/SusmithKrishnan/torghost/issues/98#issuecomment-706484941

There's also a typo in the exit command ("ecit"): https://github.com/SusmithKrishnan/torghost/blob/375bc62630cb89d1b13423c5c2b06ce79b71f0e9/build.sh#L27

Thanks for the neat tool!

[bartleskeet]

PR https://github.com/SusmithKrishnan/torghost/pull/108