Add normative MUST to evidence property.

Makes this definition closer to termsOfUse and other similar properties.

index.html

@@ -3465,7 +3465,7 @@ <h3>Evidence</h3>
         <dl>
           <dt><dfn id="defn-evidence" class="export">evidence</dfn></dt>
           <dd>
-If present, the value associated with the `evidence` [=property=] is a single
+If present, the value of the `evidence` [=property=] MUST be either a single
 object or a set of one or more objects. The following [=properties=] are defined
 for every evidence object:
             <dl>