Added bit rate example for ffprobe (#1559)

pages/common/ffprobe.md

@@ -17,3 +17,7 @@
 - Display the width or height of a video:
 
 `ffprobe -v error -select_streams v:0 -show_entries stream={{width|height}} -of default=noprint_wrappers=1:nokey=1 {{input.mp4}}`
+
+- Display the average bit rate of a video:
+
+`ffprobe -v error -select_streams v:0 -show_entries stream=bit_rate -of default=noprint_wrappers=1:nokey=1 {{input.mp4}}`