Fix command for how to build project into .jar.

jmimemagic-master  $ maven clean jar
-bash: maven: command not found

jmimemagic-master  $ ./maven clean jar
-bash: ./maven: No such file or directory

jmimemagic-master  $ mvn clean jar
...
[ERROR] Unknown lifecycle phase "jar".

jmimemagic-master  $ mvn clean jar:jar
[INFO] BUILD SUCCESS