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
kislyuk/argcomplete#212

Fixes coala#388

Users/Shell_Autocompletion.rst

@@ -18,15 +18,12 @@ If you're using ``bash``, add the following to your ``.bashrc``:
 
 ::
 
-    eval "$(register-python-argcomplete `which coala`)"
+    eval "$(register-python-argcomplete coala)"
 
 If you're using ``zsh``, add the following to your ``.zshrc``:
 
 ::
 
     autoload bashcompinit
     bashcompinit
-    eval "$(register-python-argcomplete `which coala`)"
-
-If you're seeing issues about ```which coala``` not being able to find
-``coala``, you could replace it with the path to your ``coala`` executable.
+    eval "$(register-python-argcomplete coala)"