Shell_Autocompletion.rst: Drop `which coala`
Shell_Autocompletion.rst: Drop `which coala`

The `argcomplete` package expects a script name, not a path (neither
relative, nor absolute). Using `which coala` returns a path. Instead,
use just `coala` to register with `register-python-argcomplete`.

Related `argcomplete` documentation:
https://argcomplete.readthedocs.io/en/latest/#synopsis

Related discussion with `argcomplete` devs
https://github.com/kislyuk/argcomplete/issues/212

Fixes https://github.com/coala/documentation/issues/388

coala/documentation,AnishHota/documentation,coala/documentation,rhemon/documentation,coala/documentation