Added missing using for docs

Author: mdupras

src/ImageSharp.Web/Resampler.cs

@@ -1,6 +1,8 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using SixLabors.ImageSharp.Processing;
+
 namespace SixLabors.ImageSharp.Web;
 
 /// <summary>