fix typo in doc

interface.go

@@ -7,7 +7,7 @@ import "image/color"
 // You can notice, that there are no "drawing" methods in a Target. That's because all drawing
 // happens indirectly through Triangles instance generated via MakeTriangles method.
 //
-// If no transforms are applied, the drawing are of a Target is the rectangle (-1, -1, +1, +1).
+// If no transforms are applied, the drawing area of a Target is the rectangle (-1, -1, +1, +1).
 type Target interface {
  // MakeTriangles generates a specialized copy of the provided Triangles.
  //