Fix control flow examples

[skade]

The last two examples of the control flow slides are bugged.

• Use correct code example in "return" slide
• Use working code example in "?" slide

[darnuria]

Hi, I worked on compiling code on control-flow/12.rs with trying to avoid too complicated notions I opened a pr about it! :) #24

[darnuria]

Hmm also look like there is no more example code in control flow slides involving early return the only one I see is in overview/1.rs.

control-flow/11.rs do not introduce return usage to break-out calculation and scope.