Fix avconv is not available in buster #600

Author: guysoft

src/modules/octopi/filesystem/home/pi/.octoprint/config.yaml

@@ -1,7 +1,7 @@
 webcam:
   stream: /webcam/?action=stream
   snapshot: http://127.0.0.1:8080/?action=snapshot
-  ffmpeg: /usr/bin/avconv
+  ffmpeg: /usr/bin/ffmpeg
 plugins:
   cura:
     cura_engine: /usr/local/bin/cura_engine