doc: fix overflowing literals in doc tests
doc: fix overflowing literals in doc tests

These were likely a result of copy & pasting example code. There is
actually an overflowing literal lint that would normally catch this, but
doc tests do not need to output lint warnings by default.

Rust 2018 made overflowing literals deny-by-default, and Rust 2015 just
recently did the same---even though it's a breaking change. That in turn
caused the doc tests to fail, and thus, we finally noticed it.

Fixes #144

BurntSushi/byteorder