tweaking language

the language should make it more clear that this is way how extensions
have to mapped for the XML format.

http-problem/draft-ietf-appsawg-http-problem-02.xml

@@ -613,11 +613,11 @@ Content-Language: en
 
    <t>The media type for this format is "application/problem+xml".</t>
 
-   <t>Extension arrays and objects can be serialized into the XML format by
+   <t>Extension arrays and objects are serialized into the XML format by
    considering an element containing a child or children to represent an
-   object, except for elements that contain only child element(s) named 'i',
-   which are considered arrays. For example, an alternate version of the
-   example above would appear in XML as:</t>
+   object, except for elements that contain only child element(s) named
+   'i', which are considered arrays. For example, the XML version of the
+   example above appears in XML as follows:</t>
 
      <figure><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
 HTTP/1.1 403 Forbidden