Update README to demonstrate running Selenium 3.0.0-beta3 and Firefox 48

Author: davehunt

README.md

@@ -64,11 +64,11 @@ these using the following environment variables:
 - `SELENIUM_VERSION` - version of the Selenium server
 - `FIREFOX_VERSION` - version of Firefox (only applies to Firefox images)
 
-For example, the following will start a standalone server using Selenium 2.53.1
-and Firefox 45.0.2:
+For example, the following will start a standalone server using Selenium 3.0.0-beta3
+and Firefox 48.0.2:
 
 ```bash
-$ docker run -d -p 4444:4444 -e SELENIUM_VERSION=2.53.1 -e FIREFOX_VERSION=45.0.2 selenium/standalone-firefox:2.53.1
+$ docker run -d -p 4444:4444 -e SELENIUM_VERSION=3.0.0-beta3 -e FIREFOX_VERSION=48.0.2 selenium/standalone-firefox:latest
 ```
 
 ### JAVA_OPTS Java Environment Options