Fixing ports for the example [skip ci]

Author: diemol

README.md

@@ -360,8 +360,8 @@ running inside the container.
 Here is an example with the standalone images, the same concept applies to the node images.
 ``` bash
 $ docker run -d -p 4444:4444 -p 5900:5900 -v /dev/shm:/dev/shm selenium/standalone-chrome:4.0.0-alpha-6-20200730
-$ docker run -d -p 4444:4444 -p 5901:5900 -v /dev/shm:/dev/shm selenium/standalone-firefox:4.0.0-alpha-6-20200730
-$ docker run -d -p 4444:4444 -p 5902:5900 -v /dev/shm:/dev/shm selenium/standalone-opera:4.0.0-alpha-6-20200730
+$ docker run -d -p 4445:4444 -p 5901:5900 -v /dev/shm:/dev/shm selenium/standalone-firefox:4.0.0-alpha-6-20200730
+$ docker run -d -p 4446:4444 -p 5902:5900 -v /dev/shm:/dev/shm selenium/standalone-opera:4.0.0-alpha-6-20200730
 ```
 
 Then, you would use in your VNC client: