[doc] ManagedBuffer: Fix cross-reference to isKnownUniquelyReferenced

Author: lorentey

stdlib/public/core/ManagedBuffer.swift

@@ -375,7 +375,7 @@ extension ManagedBufferPointer {
   /// Returns `true` if `self` holds the only strong reference to its
   /// buffer. Otherwise, returns `false`.
   ///
-  /// See `isUniquelyReferenced` for details.
+  /// See `isKnownUniquelyReferenced` for details.
   @inlinable
   public mutating func isUniqueReference() -> Bool {
     return _isUnique(&_nativeBuffer)