doc/muxers: fix default value for image2 option start_number.

doc/muxers.texi

@@ -794,7 +794,7 @@ ffmpeg -f v4l2 -r 1 -i /dev/video0 -f image2 -strftime 1 "%Y-%m-%d_%H-%M-%S.jpg"
 
 @table @option
 @item start_number
-Start the sequence from the specified number. Default value is 0.
+Start the sequence from the specified number. Default value is 1.
 
 @item update
 If set to 1, the filename will always be interpreted as just a