Fix slightly misleading examples.
//! ``` no_run
//! # use cortex_m::peripheral::Peripherals;
//! let mut peripherals = Peripherals::take().unwrap();
-//! peripherals.DWT.enable_cycle_counter();
+//! peripherals.DCB.enable_trace();
//! ```
//!
//! This method can only be successfully called *once* -- this is why the method returns an
//! # use cortex_m::peripheral::{DWT, Peripherals};
//! {
//! let mut peripherals = Peripherals::take().unwrap();
+//! peripherals.DCB.enable_trace();
//! peripherals.DWT.enable_cycle_counter();
//! } // all the peripheral singletons are destroyed here
//!
//! # use cortex_m::peripheral::{DWT, Peripherals};
//! {
//! let mut peripherals = Peripherals::take().unwrap();
+//! peripherals.DCB.enable_trace();
//! peripherals.DWT.enable_cycle_counter();
//! } // all the peripheral singletons are destroyed here
//!
