adjust wording in commands top-level documentation

There is no "commandline" for bindings in keys.conf, so describing it as
for "command arguments" should be more accurate.

qutebrowser/commands/__init__.py

@@ -22,7 +22,7 @@
 Some commands are hidden, which means they don't show up in the command
 completion when pressing `:`, as they're typically not useful to run by hand.
 
-In the commandline, there are also some variables you can use:
+For command arguments, there are also some variables you can use:
 
 - `{url}` expands to the URL of the current page
 - `{url:pretty}` expands to the URL in decoded format