Fix heading hierarchy consistency.

- Now the "Running the images" section consists of two alike items -- the
two ways of using this project -- standalone and hub.
- The new "Configuring the containers" section consists of two alike
  items -- JAVA_OPTS and SE_OPTS.

README.md

@@ -43,19 +43,16 @@ _Note: Only one standalone image can run on port_ `4444` _at a time._
 
 To inspect visually what the browser is doing use the `standalone-chrome-debug` or `standalone-firefox-debug` images. See [Debugging](#debugging) section for details.
 
-### Selenium Grid Hub
+### Selenium Grid Hub and Nodes
 
 ``` bash
 $ docker run -d -p 4444:4444 --name selenium-hub selenium/hub:3.0.1-fermium
-```
-
-### Chrome and Firefox Grid Nodes
-
-``` bash
 $ docker run -d --link selenium-hub:hub selenium/node-chrome:3.0.1-fermium
 $ docker run -d --link selenium-hub:hub selenium/node-firefox:3.0.1-fermium
 ```
 
+## Configuring the containers
+
 ### JAVA_OPTS Java Environment Options
 
 You can pass `JAVA_OPTS` environment variable to java process.