added priors to neg binomial example so it converges

Author: Bob Carpenter

basic_estimators/negative_binomial.stan

@@ -7,6 +7,8 @@ parameters {
   real<lower=0> beta;
 }
 model { 
+  alpha ~ cauchy(0,10);
+  beta ~ cauchy(0,10);
   for (i in 1:N)
     y[i] ~ neg_binomial(alpha, beta);
 }