Fix typo and spelling in AsyncIteratorProtocol.

Fixes rdar://78889410

(cherry picked from commit f1d87de)

Author: amartini51

stdlib/public/Concurrency/AsyncIteratorProtocol.swift

@@ -12,7 +12,7 @@
 
 import Swift
 
-/// A type that that asychronously supplies the values of a sequence one at a
+/// A type that asynchronously supplies the values of a sequence one at a
 /// time.
 ///
 /// The `AsyncIteratorProtocol` defines the type returned by the