fix a typo in readme (#191)

Author: usernameisnull

README.md

@@ -8,7 +8,7 @@
 ![Docker Automated](https://img.shields.io/docker/automated/apache/rocketmq-operator)
 [![Docker Pulls](https://img.shields.io/docker/pulls/apache/rocketmq-operator)](https://hub.docker.com/r/apache/rocketmq-operator)
 [![Docker TAG](https://img.shields.io/docker/v/apache/rocketmq-operator?label=tags&sort=date)](https://hub.docker.com/r/apache/rocketmq-operator/tags)
-![Docker Iamge](https://img.shields.io/docker/image-size/apache/rocketmq-operator)
+![Docker Image](https://img.shields.io/docker/image-size/apache/rocketmq-operator)
 [![Average time to resolve an issue](http://isitmaintained.com/badge/resolution/apache/rocketmq-operator.svg)](http://isitmaintained.com/project/apache/rocketmq-operator "Average time to resolve an issue")
 [![Percentage of issues still open](http://isitmaintained.com/badge/open/apache/rocketmq-operator.svg)](http://isitmaintained.com/project/apache/rocketmq-operator "Percentage of issues still open")
 [![Twitter Follow](https://img.shields.io/twitter/follow/ApacheRocketMQ?style=social)](https://twitter.com/intent/follow?screen_name=ApacheRocketMQ)