sync: fix broadcast link in API docs
//!
//! ## `broadcast` channel
//!
-//! The [`broadcast` channel[broadcast] supports sending **many** values from
+//! The [`broadcast` channel][broadcast] supports sending **many** values from
//! **many** producers to **many** consumers. Each consumer will receive
//! **each** value. This channel can be used to implement "fan out" style
//! patterns common with pub / sub or "chat" systems.
