fixed typo in a comment

dlib/pixel.h

@@ -801,7 +801,7 @@ namespace dlib
             else
             {
                 // perform this assignment using fixed point arithmetic: 
-                // dest = src*(alpha/255) + src*(1 - alpha/255);
+                // dest = src*(alpha/255) + dest*(1 - alpha/255);
                 // dest = src*(alpha/255) + dest*1 - dest*(alpha/255);
                 // dest = dest*1 + src*(alpha/255) - dest*(alpha/255);
                 // dest = dest*1 + (src - dest)*(alpha/255);