Update extension.rst

[issamkhadiri1989]

when using DependencyInjection\Extension\Extension, the addAnnotatedClassesToCompile is not available. so it should be mentionned that we need to use HttpKernel\DependencyInjection\Extension instead when the method addAnnotatedClassesToCompile must be used

[wouterj]

Thank you @issamkhadiri1989! Looks like Symfony's bundles also use the HttpKernel version, so this change seems 100% correct. Congratz with your first doc contribution :)

[issamkhadiri1989]

Hello @wouterj

I really appreciate your feedback and very pleased that i could contribute to this amazing doc.