test: fix compile errors (#114)

Author: toku-sa-n

src/lib.rs

@@ -30,7 +30,9 @@
 //! let mut r = unsafe { xhci::Registers::new(MMIO_BASE, mapper) };
 //! let o = &mut r.operational;
 //!
-//! o.usbcmd.update(|u| u.set_run_stop(true));
+//! o.usbcmd.update(|u| {
+//!     u.set_run_stop();
+//! });
 //! while o.usbsts.read().hc_halted() {}
 //! ```