doc: Fix ffmpeg example %f

docs/html/config_dvr.html

@@ -85,7 +85,7 @@
         <tr><td>%E</td><td>Stop time stamp of recording, UNIX epoch</td><td>1224426600</td></tr>
       </table>
       <br>
-      Example usage: /path/to/ffmpeg -i %f -vcodec libx264 -acodec copy "/path/with white space/%b"<br>
+      Example usage: /path/to/ffmpeg -i "%f" -vcodec libx264 -acodec copy "/path/with white space/%b"<br>
       You need to use quotes or escape white spaces if you want white spaces in an argument.
 
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