Fix Controller.put_flash and Presence.list arities in docs

[michallepicki]

Found warnings when using master ex_doc:

warning: documentation references Phoenix.Presence.list/1 but it doesn't exist or it's listed as @doc false (parsing Phoenix.Presence.get_by_key/3 docs)

warning: documentation references Phoenix.Presence.list/1 but it doesn't exist or it's listed as @doc false (parsing Phoenix.Presence.get_by_key/3 docs)

warning: documentation references Phoenix.Controller.put_flash/2 but it doesn't exist or it's listed as @doc false (parsing contexts docs)

This commit fixes them.

(Yay, these warnings are awesome!)

[Gazler]

Thanks <3