Change "video description" to "audio description" in 1.2.1 understanding (#3625)

for consistency/to avoid confusion

Closes #3621

Author: patrickhlauke

understanding/20/audio-only-and-video-only-prerecorded.html

@@ -43,7 +43,7 @@ <h2>Intent of Audio-only and Video-only (Prerecorded)</h2>
       <div class="note">
 
          <p>A text equivalent is not required for audio that is provided as an equivalent for
-            video with no audio information.  For example, it is not required to caption video
+            video with no audio information.  For example, it is not required to caption audio
             description that is provided as an alternative to a silent movie.
          </p>