doc: point directly to zuul.conf section

The launcher documentation about 'Starting Builds' evokes the
parameter-function in the zuul.conf documentation.  One had to goes down
a few pages until the 'Includes' subsection.  I have updated the
references to point directly to Includes, saving the reader sometime.

Also fixed a typo: paremeter -> parameter

Change-Id: I560f857b40a7cb989a71161f94e1fb0c8fd69293
diff --git a/doc/source/launchers.rst b/doc/source/launchers.rst
index cefd582..d8145ab 100644
--- a/doc/source/launchers.rst
+++ b/doc/source/launchers.rst
@@ -164,7 +164,8 @@
 
 where **NODE_NAME** is the name or class of node on which the job
 should be run.  This can be specified by setting the ZUUL_NODE
-parameter in a paremeter-function (see :ref:`zuulconf`).
+parameter in a parameter-function (see :ref:`includes` section in
+:ref:`zuulconf`).
 
 Zuul sends the ZUUL_* parameters described in `Zuul Parameters`_
 encoded in JSON format as the argument included with the