Fix example in FFI tutorial.

[ejholmes]

These changes were necessary to get this example to compile. Not sure how to get command line args now, since fn main(args: ~[~str]) produces the following compiler error:

main.rs:21:0: 23:1 error: Wrong type in main function: found `fn(~[~str])`, expected `fn() -> ()`

[catamorphism]

You can get command-line args by calling core::os::args. If you don't mind changing that, I'll merge. Thanks! Also, can you change the target branch to incoming instead of master? (This may require closing this and submitting a new pull request, I'm not sure.)