[rand.dist.bern.negbin] Fix index for k member

Author: jwakely

source/numerics.tex

@@ -4776,7 +4776,7 @@
  correspond to the respective parameters of the distribution.
 \end{itemdescr}
 
-\indexlibrarymember{t}{negative_binomial_distribution}%
+\indexlibrarymember{k}{negative_binomial_distribution}%
 \begin{itemdecl}
 IntType k() const;
 \end{itemdecl}