The example code uses trigger_callback(), not do_callback().
Use a space after colons per the Rust coding style:
https://github.com/rust-lang/rust-guidelines/blob/master/style/whitespace.md
auto merge of #17187 : damag/rust/ffi-guide-fixes, r=alexcrichton

Updates the callbacks section to refer to the right function name and fixes a couple of minor whitespace issues in the examples.