Minor update to VDOT Marathon Plans

I've corrected a minor mistake in my VDOT calculator. Week 20 of the Jack Daniel's Marathon Plan 'A' was set to 10 miles T when it should have been 10 miles E. This is an error in the early copies of the second edition of the book. Thanks to James B for pointing this out. http://fellrnr.com/wiki/VDOT_Calculator