Fix example usage of -debug flag

The CLI changed and now wants the `-debug` flag to be passed with a boolean value.

See SeleniumHQ/selenium@705be71.

Author: jamesottaway

README.md

@@ -67,7 +67,7 @@ $ docker run -d -p 4444:4444 -e JAVA_OPTS=-Xmx512m --name selenium-hub selenium/
 You can pass `SE_OPTS` variable with additional commandline parameters for starting a hub or a node.
 
 ``` bash
-$ docker run -d -p 4444:4444 -e SE_OPTS=-debug --name selenium-hub selenium/hub:3.2.0-actinium
+$ docker run -d -p 4444:4444 -e SE_OPTS="-debug true" --name selenium-hub selenium/hub:3.2.0-actinium
 ```
 
 ### PHANTOMJS_OPTS PhantomJS Configuration Options