fix comments

src/ImageSharp/Common/Memory/BufferPointer{T}.cs

@@ -73,16 +73,16 @@ namespace ImageSharp
         /// </summary>
         public IntPtr PointerAtOffset { get; private set; }
 
-          /// <summary>
-        /// Gets or sets the pixel at the specified position.
+        /// <summary>
+        /// Gets the element at the specified position.
         /// </summary>
-        /// <param name="x">The x-coordinate of the pixel. Must be greater than or equal to zero and less than the width of the image.</param>
-        /// <returns>The <see typeparam="TColor"/> at the specified position.</returns>
-        public T this[int x]
+        /// <param name="index">The index from the start of this Pointer to the required element.</param>
+        /// <returns>The <see typeparam="T"/> at the specified position.</returns>
+        public T this[int index]
         {
             get
             {
-                void* ptr = ((byte*)this.PointerAtOffset) + (x * Unsafe.SizeOf<T>());
+                byte* ptr = ((byte*)this.PointerAtOffset) + BufferPointer.SizeOf<T>(index);
                 return Unsafe.Read<T>(ptr);
             }
         }