Fixing build warning around a cref for a type missing in PL.

Rx.NET/Source/System.Reactive.Core/Reactive/Concurrency/LocalScheduler.TimerQueue.cs

@@ -99,7 +99,7 @@ namespace System.Reactive.Concurrency
         private static readonly TimeSpan RETRYSHORT = TimeSpan.FromMilliseconds(50);
 
         /// <summary>
-        /// Longest interval supported by <see cref="System.Threading.Timer"/>.
+        /// Longest interval supported by System.Threading.Timer.
         /// </summary>
         private static readonly TimeSpan MAXSUPPORTEDTIMER = TimeSpan.FromMilliseconds((1L << 32) - 2);