Fix: Use correct interface for custom extensions (missing one)

Author: CharrafiMed

docs/2.6/customization/extensions.md

@@ -9,7 +9,7 @@ redirect_from: /customization/extensions/
 
 Extensions provide a way to group related parsers, renderers, etc. together with pre-defined priorities, configuration settings, etc.  They are perfect for distributing your customizations as reusable, open-source packages that others can plug into their own projects!
 
-To create an extension, simply create a new class implementing `ExtensionInterface`.  This has a single method where you're given a `ConfigurableEnvironmentInterface` to register whatever things you need to. For example:
+To create an extension, simply create a new class implementing `ExtensionInterface`.  This has a single method where you're given a `EnvironmentBuilderInterface` to register whatever things you need to. For example:
 
 ```php
 use League\CommonMark\Extension\ExtensionInterface;