message: Fix typo in rustdoc
diff: //! let _sampler = meter.sampler();
  //! }
  //!
  -//! // Get the moving average. The meter tracks a fixed number of samles, and
  +//! // Get the moving average. The meter tracks a fixed number of samples, and
  //! // the average won't mean much until it's filled up at least once.
  //! println!("Average time: {}", meter.average());
  