README.md update for activemq username (#2959)
README.md update for activemq username (#2959)

modified username field for ACTIVEMQ to match zipkin-server-shared.yml
openzipkin/zipkin,adriancole/zipkin,openzipkin/zipkin,twitter/zipkin,twitter/zipkin,twitter/zipkin,twitter/zipkin,adriancole/zipkin,openzipkin/zipkin,openzipkin/zipkin,adriancole/zipkin,openzipkin/zipkin,adriancole/zipkin