Update README.adoc: don't use tty for docker output

For streaming output, audiowmark uses stdout for the watermarked audio
wav and stderr for status information. Using the -t ("tty") option for
docker mixes both streams which breaks streaming output.
Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

README.adoc

@@ -574,4 +574,4 @@ You should be able to execute `audiowmark` via Docker.
 Example that outputs the usage message:
 
   docker build -t audiowmark .
-  docker run -v <local-data-directory>:/data --rm -it audiowmark -h
+  docker run -v <local-data-directory>:/data --rm -i audiowmark -h