Update index.rst

To add a custom stimulus controller to the root component element the name can not contain `-controller`.

If using `some_custom_controller.js`, the name should be `some-custom`

I had some issues with that and figured it out on this github issue : #687 (comment)

Author: DennisdeBest

src/LiveComponent/doc/index.rst

@@ -880,7 +880,7 @@ To add a custom Stimulus controller to your root component element:
 
 .. code-block:: html+twig
 
-    <div {{ attributes.defaults(stimulus_controller('my-controller', { someValue: 'foo' })) }}>
+    <div {{ attributes.defaults(stimulus_controller('some-custom', { someValue: 'foo' })) }}>
 
 JavaScript Component Hooks
 ~~~~~~~~~~~~~~~~~~~~~~~~~~