use must be initialized wording over should

spec.html

@@ -49674,7 +49674,7 @@ <h1>%Symbol.toStringTag%</h1>
     <h1>Module Source Objects</h1>
     <p>Module Source Objects represent modules in their source import phase, which are not linked, instantiated or executed.</p>
     <p>A <dfn>Module Source Object</dfn> is an object for which HostGetModuleSourceName returns a normal completion.</p>
-    <p>All Module Source Objects should have a prototype of %AbstractModuleSource%.prototype.</p>
+    <p>All Module Source Objects must be initialized with a prototype of %AbstractModuleSource%.prototype.</p>
     <p>Hosts may define their own %AbstractModuleSource% subclasses for custom module types.</p>
 
     <emu-clause id="sec-abstractmodulesource-constructor">