Fix typo (dotnet/coreclr#15129)

Signed-off-by: dotnet-bot <[email protected]>

Commit migrated from dotnet/coreclr@44ad761

src/coreclr/src/mscorlib/shared/System/IO/MemoryStream.cs

@@ -2,7 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
 using System.Threading;
@@ -504,7 +503,7 @@ public override void CopyTo(Stream destination, int bufferSize)
 
         public override Task CopyToAsync(Stream destination, int bufferSize, CancellationToken cancellationToken)
         {
-            // This implementation offers beter performance compared to the base class version.
+            // This implementation offers better performance compared to the base class version.
 
             StreamHelpers.ValidateCopyToArgs(this, destination, bufferSize);