[BMI776] Correction to Jukes-Cantor substitution matrix

[Colin Dewey]

Hi BMI 776ers,

I realized over the weekend that there is an error in the slides and  
notes that I passed out regarding the form of the Jukes-Cantor  
matrix.  In the matrix P(t), all occurrences of $-4 \lambda t$ should  
be replaced by $-\lambda t$ (i.e., just remove the factor of 4).   
This presentation is slightly different than that in the text (which  
uses a parameter \alpha), but the two are easily related by $\lambda  
= 4 \alpha$.

This correction is critical for your HW assignment, so please check  
your code.  I've updated the slides for lecture 4 on the Web site.

- Colin