Apply Harlan's suggestions from code review

Co-authored-by: Harlan Haskins <[email protected]>

Author: invalidname

stdlib/public/Concurrency/AsyncStream.swift

@@ -44,7 +44,7 @@ import Swift
 ///
 /// Consider a hypothetical `QuakeMonitor` type that provides callers with
 /// `Quake` instances every time it detects an earthquake. To receive callbacks,
-/// callers set a custom closure a closure as the value of the monitor's
+/// callers set a custom closure as the value of the monitor's
 /// `quakeHandler` property, which the monitor calls back as necessary.
 ///
 ///     class QuakeMonitor {