Fix ffmpeg plugin readme, "ac" -> "ach"

ecosystem/ffmpeg_plugin/README.md

@@ -152,7 +152,7 @@ ffmpeg -stream_loop -1 -video_size 1920x1080 -f rawvideo -pix_fmt yuv422p10le -i
 Reading a st2110-30 stream(pcm24,1ms packet time,2 channels) on "239.168.85.20:30000" with payload_type 111 and encoded to a wav file:
 
 ```bash
-ffmpeg -p_port 0000:af:01.0 -p_sip 192.168.96.2 -p_rx_ip 239.168.85.20 -udp_port 30000 -payload_type 111 -pcm_fmt pcm24 -at 1ms -ac 2 -f mtl_st30p -i "0" dump.wav -y
+ffmpeg -p_port 0000:af:01.0 -p_sip 192.168.96.2 -p_rx_ip 239.168.85.20 -udp_port 30000 -payload_type 111 -pcm_fmt pcm24 -at 1ms -ach 2 -f mtl_st30p -i "0" dump.wav -y
 ```
 
 ### 4.2 St30p output
@@ -170,7 +170,7 @@ For pcm16 audio, use `mtl_st30p_pcm16` muxer, set `pcm_fmt` to `pcm16` for demux
 ```bash
 ffmpeg -stream_loop -1 -i test.wav -p_port 0000:af:01.1 -p_sip 192.168.96.3 -p_tx_ip 239.168.85.20 -udp_port 30000 -payload_type 111 -at 1ms -f mtl_st30p_pcm16 -
 
-ffmpeg -p_port 0000:af:01.0 -p_sip 192.168.96.2 -p_rx_ip 239.168.85.20 -udp_port 30000 -payload_type 111 -pcm_fmt pcm16 -at 1ms -ac 2 -f mtl_st30p -i "0" dump_pcm16.wav -y
+ffmpeg -p_port 0000:af:01.0 -p_sip 192.168.96.2 -p_rx_ip 239.168.85.20 -udp_port 30000 -payload_type 111 -pcm_fmt pcm16 -at 1ms -ach 2 -f mtl_st30p -i "0" dump_pcm16.wav -y
 ```
 
 ### Enabling experimental MTL_GPU_DIRECT in FFmpeg with ST20p Support