Update Symfony.php

Adding link to central "Parts" chapter, as announced in Codeception/module-webdriver#33 (comment)
If you agree, I'll copy-paste this to all other modules that have parts.

Author: ThomasLandauer

src/Codeception/Module/Symfony.php

@@ -78,7 +78,10 @@
  *
  * ## Parts
  *
- * * services - allows to use Symfony DIC only with WebDriver or PhpBrowser modules.
+ * * `services`: Symfony dependency injection container (DIC)
+ *
+ * See [WebDriver module](https://codeception.com/docs/modules/WebDriver#Loading-Parts-from-other-Modules)
+ * for general information on how to load parts of a framework module.
  *
  * Usage example:
  *
@@ -87,12 +90,12 @@
  * modules:
  *     enabled:
  *         - Symfony:
- *             part: SERVICES
+ *             part: services
  *         - Doctrine2:
  *             depends: Symfony
  *         - WebDriver:
  *             url: http://your-url.com
- *             browser: phantomjs
+ *             browser: firefox
  * ```
  *
  */