Update extending.md

Fix typo.

Signed-off-by: Christopher Frost <cfrost@pivotal.io>

Author: mmc2004jp

docs/extending.md

@@ -1,7 +1,7 @@
 # Extending
 For general information on extending the buildpack, refer to [Configuration and Extension](../README.md#configuration-and-extension).
 
-To add a component, its class name must be added added to [`config/components.yml`][].  It is recommended, but not required, that the class' file be placed in a directory that matches its type.
+To add a component, its class name must be added to [`config/components.yml`][].  It is recommended, but not required, that the class' file be placed in a directory that matches its type.
 
 | Component Type | Location
 | -------------- | --------