Editorial: Add Intl.DurationFormat to list of constructors

Author: linusg

spec/intl.html

@@ -39,6 +39,11 @@ <h1>Intl.DisplayNames ( . . . )</h1>
       <p>See <emu-xref href="#intl-displaynames-objects"></emu-xref>.</p>
     </emu-clause>
 
+    <emu-clause id="sec-intl.durationformat-intro">
+      <h1>Intl.DurationFormat ( . . . )</h1>
+      <p>See <emu-xref href="#durationformat-objects"></emu-xref>.</p>
+    </emu-clause>
+
     <emu-clause id="sec-intl.listformat-intro">
       <h1>Intl.ListFormat ( . . . )</h1>
       <p>See <emu-xref href="#listformat-objects"></emu-xref>.</p>