Add copy/pasta example for system preferences.

Most of the other api docs have one example of where to find the module in a copy/paste format. This pull request adds one for the system preferences.

docs/api/system-preferences.md

@@ -2,6 +2,11 @@
 
 > Get system preferences.
 
+```javascript
+const {systemPreferences} = require('electron');
+console.log(systemPreferences.isDarkMode());
+```
+
 ## Methods
 
 ### `systemPreferences.isDarkMode()` _macOS_