Adding spider food for SEO purposes

Adding the information about more advanced configuration syntax for `Injector` for the purpose of more easily finding documentation regarding use of percent and dollar signs (particularly when encountered in YAML configs, albeit it’s not restricted only to YAML configs).

> References are denoted by by using a percent and dollar sign, like in the  YAML configuration example below.

Right now at the time of writing, it’s pretty difficult to surface information about this mysterious syntax by googling searches like “silverstripe percent dollar sign config” and may be confusing to beginners who are encountering this syntax in existing projects. 😊 I know it would have helped me!

docs/en/02_Developer_Guides/05_Extending/05_Injector.md

@@ -276,7 +276,8 @@ By default, services registered with Injector do not inherit from one another; T
 named services, which may not be actual classes, and thus should not behave as though they were.
 
 Thus if you want an object to have the injected dependencies of a service of another name, you must
-assign a reference to that service.
+assign a reference to that service. References are denoted by by using a percent and dollar sign, like in the 
+YAML configuration example below.
 
 ```yaml
 SilverStripe\Core\Injector\Injector: