fix(docs): fix stopper.stop wrong variable name. closes #2244

Was:

```js
var stopper = require('karma').stopper
runner.stop({port: 9876}, function(exitCode) {
```

Changed to :

```js
var stopper = require('karma').stopper
stopper.stop({port: 9876}, function(exitCode) {
```

docs/dev/04-public-api.md

@@ -126,7 +126,7 @@ This function will signal a running server to stop.  The equivalent of `karma st
 
 ```javascript
 var stopper = require('karma').stopper
-runner.stop({port: 9876}, function(exitCode) {
+stopper.stop({port: 9876}, function(exitCode) {
   if (exitCode === 0) {
     console.log('Server stop as initiated')
   }
@@ -136,4 +136,4 @@ runner.stop({port: 9876}, function(exitCode) {
 
 ## Callback function notes
 
-- If there is an error, the error code will be provided as the second parameter to the error callback.
+- If there is an error, the error code will be provided as the second parameter to the error callback.