[doc] fix mapping in get it started 

seems like the pass same for payum/payum and payum/core packages. Let's simplify doc than.

fixes https://github.com/Payum/PayumBundle/issues/109
Revert "[doc] fix mapping in get it started "

This reverts commit 7d6c7070fe19db1d749de363e38080884bb3a51e.
[doc] fix mapping path when `payun/core` package only used.

it fixes https://github.com/Payum/PayumBundle/issues/109
Merge branch '0.8'