Modal Dialog Pattern: Clarify First Note in States and Properties Subsection

Per feedback from @a11ydoer in issue #325, changed:

> Because marking a dialog modal by setting aria-modal
> to true can prevent users of some assistive technologies from perceiving content outside the dialog,
> users of those technologies will experience severe negative ramifications if a dialog is marked modal but is not modal for other users.

To:

> Because marking a dialog modal by setting aria-modal
> to true can prevent users of some assistive technologies from perceiving content outside the dialog,
> users of those technologies will experience severe negative ramifications if a dialog is marked modal but does not behave as a modal for other users.

Author: mcking65

aria-practices.html

@@ -580,7 +580,7 @@ <h4>WAI-ARIA Roles, States, and Properties</h4>
             Because marking a dialog modal by setting 
             <a href="#aria-modal" class="property-reference">aria-modal</a>
             to <code>true</code> can prevent users of some assistive technologies from perceiving content outside the dialog,
-            users of those technologies will experience severe negative ramifications if a dialog is marked modal but is not modal for other users.
+            users of those technologies will experience severe negative ramifications if a dialog is marked modal but does not behave as a modal for other users.
             So, mark a dialog modal <strong>only when:</strong>
             <ol>
               <li>Application code prevents all users from interacting in any way with content outside of it.</li>