Fix documentation to match usb-device 0.3

[kpcyrd]

UsbDeviceBuilder has changed slightly, this updates the documentation accordingly. Also it's now possible to run tests:

% cargo test              
   Compiling usbd-serial v0.2.2 (/redacted/usbd-serial)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.29s
     Running unittests src/lib.rs (target/debug/deps/usbd_serial-8f07a6ca69583646)

running 4 tests
test buffer::tests::clear ... ok
test buffer::tests::discard ... ok
test buffer::tests::read ... ok
test buffer::tests::write ... ok

test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests usbd_serial

running 1 test
test src/lib.rs - (line 16) - compile ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.03s

[eldruin]

Looks good to me, thank you!