fix error

Author: as6325400

docker/README.md

@@ -266,7 +266,7 @@ To build domjudge with local sources, run
   dj_packaging_dir=<path to domjudge-packaging>
   dj_source_dir =<domjudge source directory>
   tar --exclude-vcs -czf "${dj_packaging_dir}/docker/domjudge.tar.gz" -C $(dirname "$dj_source_dir") $(basename "$dj_source_dir")
-  cd ${dj_packaging_dir}/docker"
+  cd ${dj_packaging_dir}/docker
   docker build -t domjudge -f domserver/Dockerfile .
 ```
 Note that the source directory name has to match `domjudge-*`.