ACTION-1548: Additional update to aria-current

1. Turn "user agents" into "user agents or assistive technologies" in the
   first normative statement.
2. Turn "MUST" into "SHOULD" in that same statement.

https://lists.w3.org/Archives/Public/public-pfwg/2015Mar/0053.html

aria/aria.html

@@ -7986,7 +7986,7 @@ <h2>Definitions of States and Properties (all aria-* attributes)</h2>
 			<sdef>aria-current</sdef>
 			<div class="state-description">
 				<p>[ARIA 1.1] Indicates the <a title="element">element</a> that represents the current item within a container or set of related elements.</p>
-				<p>The <aref>aria-current</aref> <a>attribute</a> is an enumerated type. Any <a>value</a> not included in the list of allowed values MUST be treated by <a title="user agent">user agents</a> as if the value <code>true</code> had been provided. If the attribute is not present or its value is an empty string, the default value of <code>false</code> applies and the <aref>aria-current</aref> <a>state</a> MUST NOT be exposed by user agents or <a>assistive technologies</a>.</p>
+				<p>The <aref>aria-current</aref> <a>attribute</a> is an enumerated type. Any <a>value</a> not included in the list of allowed values SHOULD be treated by <a title="user agent">user agents</a> or <a>assistive technologies</a> as if the value <code>true</code> had been provided. If the attribute is not present or its value is an empty string, the default value of <code>false</code> applies and the <aref>aria-current</aref> <a>state</a> MUST NOT be exposed by user agents or assistive technologies.</p>
 				<p>The <aref>aria-current</aref> attribute is used when an element within a set of related elements is visually styled to indicate it is the current item in the set. For example:</p>
 				<ul>
 					<li>A <code>page</code> token used to indicate a link within a set of pagination links, where the link is visually styled to represent the currently-displayed page.</li>