Add print statement in control flow example

[tomek7667]

The guided tour example doesn't specify print after the greeting, so the experiment that follows won't be valid:

Change optionalName to nil. What greeting do you get? Add an else clause that sets a different greeting if optionalName is nil.

[amartini51]

Thanks for pointing this out! The examples throughout the tour use "bare" expressions in several places, expecting that the reader is running the code in a REPL or playground, so the results of function calls are visible even without printing. Older versions of the book included a link to download the chapter as a playground, which was lost when moving to DocC (see #33). However, looking again today, DocC now supports a @CallToAction directive, which should let us restore that download-playground functionality.

With that in mind, I think we're better off fixing the reader in general, instead of spot-fixing one specific example. Or, if it turns out we can't use @CallToAction, the tour should be revised to include explicit print() everywhere the code produces a result that otherwise wouldn't be visible.