Fix a mistake in TextLayoutBuilder's README

Summary:
@public This fixes #6 by fixing the README file, in which
`super.draw()` was placed instead of `super.onDraw()`.

Reviewed By: sriramramani

Differential Revision: D4366906

fbshipit-source-id: 3fc283545b8bc8d1fb9bbfa9e592a252273b5d46

README.md

@@ -60,7 +60,7 @@ Usage
 
       @Override
       protected void onDraw(Canvas canvas) {
-          super.draw(canvas);
+          super.onDraw(canvas);
 
           // Draw the layout.
           mLayout.draw(canvas);