Fix typos in docker/Dockerfile

Fix "one" -> "done", and add missing closing parenthesis.

Author: slel

docker/Dockerfile

@@ -14,7 +14,7 @@ LABEL maintainer="Etherpad team, https://github.com/ether/etherpad-lite"
 ARG ETHERPAD_VERSION=develop
 
 # Set the following to production to avoid installing devDeps
-# this can be one with build args (and is mandatory to build ARM version
+# this can be done with build args (and is mandatory to build ARM version)
 ARG NODE_ENV=development
 
 # grab the ETHERPAD_VERSION tarball from github (no need to clone the whole