small fix for document

RPing · RPing · commit 6b5a1f261294 · 2018-09-16T01:16:11.000+08:00
diff --git a/README.md b/README.md
@@ -41,7 +41,7 @@ Take for example a signal graph that looks like this:
 
 The corresponding command-line arguments are pretty gnarly:
 ```bash
-ffmpeg -i input.mp4 -filter_complex "[0]trim=start_frame=10:end_frame=20[v0];\
+ffmpeg -i input.mp4 -i overlay.png -filter_complex "[0]trim=start_frame=10:end_frame=20[v0];\
     [0]trim=start_frame=30:end_frame=40[v1];[v0][v1]concat=n=2[v2];[1]hflip[v3];\
     [v2][v3]overlay=eof_action=repeat[v4];[v4]drawbox=50:50:120:120:red:t=5[v5]"\
     -map [v5] output.mp4
