Reword schema intro

Author: ruffsl

articles/ros2_access_control_policies.md

@@ -83,8 +83,8 @@ In XML, this is achieved using [XInclude](https://en.wikipedia.org/wiki/XInclude
 
 ## Schema
 
-This section is a description of a SROS 2 policy schema, expressed in terms of constraints on the structure and content of the document that are beyond the basic syntactic constraints imposed by XML itself.
-This schema and accompanying definitions for grammatical rules governing the structure and content for elements and attributes is described below.
+The SROS 2 policy schema is defined with XML.
+The elements and attributes that make up a policy are described below.
 
 ``` xml
 {% include_relative ros2_access_control_policies/policy.xsd %}