Fix Entity namespace alias

[xelaris]

Q	A
Doc fix?	yes
New docs?	no
Applies to	all
Fixed tickets

Change the Entity namespace alias from App:Post to AppBundle:Post and use shortcut method so make the code simpler and equal to the newAction later in the chapter

[xabbuh]

Thank you for reading the docs so carefully and providing good patches @xelaris! However, this change will already be included in #4695. So I'm going to close here.