Bug 1172020 - Docs: Remove port 8000 mention on installation docs

Since gunicorn is only running on port 8000 inside the VM, it's then
proxied via varnish/apache and accessible on port 80 outside the VM, so
mentioning port 8000 is more confusing than helpful.

docs/installation.rst

@@ -90,14 +90,12 @@ Setting up a local Treeherder instance
 Viewing the local server
 ------------------------
 
-* Start a gunicorn instance listening on port 8000
+* Start a gunicorn instance, to serve API requests:
 
   .. code-block:: bash
 
      (venv)vagrant@local:~/treeherder$ ./bin/run_gunicorn
 
-  all the request sent to local.treeherder.mozilla.org will be proxied to it by varnish/apache.
-
 * Or for development you can use the django runserver instead of gunicorn:
 
   .. code-block:: bash