Change "sidebar" to "client" in config.md

config.md was interchangeably using "sidebar" and "client" to refer to
the client. Fix it to always use "client" because it's more consistent.
I chose client instead of sidebar because (both technically speaking and
in terms of the UI) not all of these settings actually refer to the
sidebar, e.g. showHighlights and assetRoot. Also client is the word used
by the git repo name and in the README file.

docs/config.md

-Configuring the sidebar
-=======================
+Configuring the client
+======================
 
-The Hypothesis sidebar can be configured by providing a settings object in the
+The Hypothesis client can be configured by providing a settings object in the
 body of the hosting page:
 
 ```html