docs(docker): fix env name error

deploy/docker/README.md

@@ -83,7 +83,7 @@ These environment variables will ignore for configuration file.
 
 The list is incomplete and may changed with [etc/emqx.conf](https://github.com/emqx/emqx/blob/master/etc/emqx.conf) and plugin configuration files. But the mapping rule is similar.
 
-If set ``EMQX_NAME`` and ``EMQX_HOST``, and unset ``EMQX_NODE__NAME``, ``EMQX_NODE__NAME=$EMQX_NAME@$EMQX_HOST``.
+If set ``EMQX_NAME`` and ``EMQX_HOST``, and unset ``EMQX_NODE_NAME``, ``EMQX_NODE_NAME=$EMQX_NAME@$EMQX_HOST``.
 
 For example, set mqtt tcp port to 1883