typo fix

Co-authored-by: youennf <[email protected]>

getusermedia.html

@@ -5752,7 +5752,7 @@ <h2>Defining a new constrainable property</h2>
       aware of in an attempt to reduce potential usage conflicts.</p>
     </section>
     <section>
-      <h2>Defining new a sink for {{MediaStreamTrack}} and {{MediaStream}}</h2>
+      <h2>Defining a new sink for {{MediaStreamTrack}} and {{MediaStream}}</h2>
       <p>It is likely that new sinks for {{MediaStream}} and/or {{MediaStreamTrack}}
       will be defined in the future. At a minimum, a new consumer of a
       {{MediaStreamTrack}} will need to define:</p>