Bug in sketch for Example 8.9

[sidwellr]

The sketch for Example 8.9 (when "Open in Web Editor" is clicked) has a bug: 'G' is supposed to move the turtle without drawing a line, but the code treats 'F' and 'G' the same. The code snippet in the book just before the example is correct. The particular L-system used in example 8.9 doesn't have a 'G', so it works OK. But changing the code to use an L-system which contains 'G' won't work.