Delete stray word from docs.

Fixes rdar://78890316

Author: amartini51

stdlib/public/Concurrency/PartialAsyncTask.swift

@@ -243,7 +243,7 @@ internal func _resumeUnsafeThrowingContinuationWithError<T>(
 #endif
 
 /// Suspends the current task,
-/// then calls the given closure with the an unsafe continuation for the current task.
+/// then calls the given closure with an unsafe continuation for the current task.
 ///
 /// - Parameter fn: A closure that takes an `UnsafeContinuation` parameter.
 /// You must resume the continuation exactly once.
@@ -260,7 +260,7 @@ public func withUnsafeContinuation<T>(
 }
 
 /// Suspends the current task,
-/// then calls the given closure with the an unsafe throwing continuation for the current task.
+/// then calls the given closure with an unsafe throwing continuation for the current task.
 ///
 /// - Parameter fn: A closure that takes an `UnsafeContinuation` parameter.
 /// You must resume the continuation exactly once.