dfn fixes

Co-authored-by: Dominique Hazael-Massieux <[email protected]>

Author: fippo

Diff for: index.bs

@@ -358,7 +358,7 @@ dictionary RTCEncodedVideoFrameMetadata {
         </p>
     </dd>
     <dt>
-        <dfn>mimeType</dfn> of type <span class="idlMemberType">DOMString</span>
+        <dfn dict-member>mimeType</dfn> of type <span class="idlMemberType">DOMString</span>
     </dt>
     <dd>
         <p>
@@ -481,7 +481,7 @@ dictionary RTCEncodedAudioFrameMetadata {
         </p>
     </dd>
     <dt>
-        <dfn>mimeType</dfn> of type <span class="idlMemberType">DOMString</span>
+        <dfn dict-member>mimeType</dfn> of type <span class="idlMemberType">DOMString</span>
     </dt>
     <dd>
         <p>