runtime: fix typo in the task modules
//!
//! The task uses a reference count to keep track of how many active references
//! exist. The Unowned reference type takes up two ref-counts. All other
-//! reference types take pu a single ref-count.
+//! reference types take up a single ref-count.
//!
//! Besides the waker type, each task has at most one of each reference type.
//!
