Fix a doctest; second one to be fixed
//! let probes = Probe::list_all();
//!
//! // Use the first probe found.
-//! let probe = probes[0].open()?;
+//! let mut probe = probes[0].open()?;
//!
//! // Attach to a chip.
-//! let session = probe.attach("nrf52")?;
+//! let mut session = probe.attach("nrf52")?;
//!
//! // Select a core.
-//! let core = session.attach_to_core(0)?;
+//! let mut core = session.attach_to_core(0)?;
//!
//! // Halt the attached core.
//! core.halt()?;
