Fix problems with example inventory in the intro_patterns doc (#1138)

* invalid inventory format in patterns doc

The example inventory placed the host alias directly in the
`groups['atlanta']` dictionary, instead of in the `groups['atlanta']['hosts']`
dictionary required by the inventory format.

* Fix misleading hostvar name in example

The documentation implies that ansible uses `hostvars['host1']['host']` to
store the address of a target host (as specified in the example inventory),
but in fact it would be stored in `hostvars['host1']['ansible_host']`. This
is confusing for newcomers.

Author: dobbymoodge

docs/docsite/rst/inventory_guide/intro_patterns.rst

@@ -111,10 +111,11 @@ Your pattern must match your inventory syntax. If you define a host as an :ref:`
 .. code-block:: yaml
 
     atlanta:
-      host1:
-        http_port: 80
-        maxRequestsPerChild: 808
-        host: 127.0.0.2
+      hosts:
+        host1:
+          http_port: 80
+          maxRequestsPerChild: 808
+          ansible_host: 127.0.0.2
 
 you must use the alias in your pattern. In the example above, you must use ``host1`` in your pattern. If you use the IP address, you will once again get the error: