[stdlib][docs] fix the SeeAlso documentation for withDiscardingTaskGroup

The SeeAlso documentation for withDiscardingTaskGroup is missing trailing backticks.

Author: kkebo

stdlib/public/Concurrency/DiscardingTaskGroup.swift

@@ -66,7 +66,7 @@ import Swift
 /// For tasks that need to handle cancellation by throwing an error,
 /// use the `withThrowingDiscardingTaskGroup(returning:body:)` method instead.
 ///
-/// - SeeAlso: ``withThrowingDiscardingTaskGroup(returning:body:)
+/// - SeeAlso: ``withThrowingDiscardingTaskGroup(returning:body:)``
 @available(SwiftStdlib 5.9, *)
 @inlinable
 @_unsafeInheritExecutor