Update docs for construct_homepage_panels hook (#5109)

`construct_homepage_panels` hook doesn't need a return statement.

[Looking at the code](https://github.com/wagtail/wagtail/blob/master/wagtail/admin/views/home.py#L101-L102), the user doesn't need to return a list, they need to mutate the list passed in.

CONTRIBUTORS.rst

@@ -349,6 +349,7 @@ Contributors
 * Thomas Elliott
 * damianosSemmle
 * Evan Winter
+* Neil Lyons
 
 Translators
 ===========

docs/reference/hooks.rst

@@ -76,7 +76,7 @@ Hooks for building new areas of the admin interface (alongside pages, images, do
 
     @hooks.register('construct_homepage_panels')
     def add_another_welcome_panel(request, panels):
-      return panels.append( WelcomePanel() )
+        panels.append(WelcomePanel())
 
 
 .. _construct_homepage_summary_items: