docs(echo): mark the sample clip step as optional

The prose said "make one if you have nothing to hand" while the block
made one unconditionally. Put the condition on the line it applies to.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: rickstaa

echo/README.md

@@ -42,10 +42,10 @@ curl -sk https://localhost:8935/discovery | jq '.[].runners[].app'   # confirm l
 
 The input is a file path, or `-` to read an MPEG-TS stream from stdin (so you can pipe in anything ffmpeg produces); the output is a file, or `-` to write the echoed stream to stdout (pipe it to a player).
 
-**From a file** — writes the result to `echo-out.ts`. Any video works; make one if you have nothing to hand (`-t` sets the length):
+**From a file** — writes the result to `echo-out.ts`. Any video works; the first command makes a 30s one (`-t` sets the length):
 
 ```sh
-ffmpeg -f lavfi -i testsrc=size=1280x720:rate=30 -t 30 -c:v libx264 -preset ultrafast -pix_fmt yuv420p sample.mp4
+ffmpeg -f lavfi -i testsrc=size=1280x720:rate=30 -t 30 -c:v libx264 -preset ultrafast -pix_fmt yuv420p sample.mp4   # skip if you have a video
 uv run client.py --mode blur --discovery https://localhost:8935/discovery sample.mp4
 ```