Fix CDF of F distribution

The CDF formula of the F distribution contained a typo where d_1 got
repeated twice instead of being once d_1 and once d_2.

Reported by Catherine Tousey.

stat-cookbook.tex

@@ -37,7 +37,7 @@
 %     improvements.
 %   - The patch version changes for trivial fixes, such as typos in text or
 %     formulas.
-\newcommand{\version}{0.2.4}
+\newcommand{\version}{0.2.5}
 
 % Probability and Statistics LaTeX shortcuts.
 \input{probstat}
@@ -252,7 +252,7 @@ \subsection{Continuous Distributions}
   (1-2s)^{-k/2} \; s<1/2\\[3ex]
 
   F & \text{F}(d_1,d_2) &
-  I_\frac{d_1x}{d_1x+d_2}\left(\frac{d_1}{2},\frac{d_1}{2}\right) &
+  I_\frac{d_1x}{d_1x+d_2}\left(\frac{d_1}{2},\frac{d_2}{2}\right) &
   \frac{\sqrt{\frac{(d_1x)^{d_1} d_2^{d_2}}{(d_1x+d_2)^{d_1+d_2}}}}
     {x\mathrm{B}\left(\frac{d_1}{2},\frac{d_1}{2}\right)} &
   \frac{d_2}{d_2-2} %\; d_2 > 2