fix typo in comment

Signed-off-by: Paul Wilson <[email protected]>

src/random_number_generator.h

@@ -179,7 +179,7 @@ class ExponentialDoubleDist : public DoubleDistribution {
 };
 
 
-/// Binary distribution requires twp options and a probability
+/// Binary distribution requires two options and a probability
 class BinaryDoubleDist : public DoubleDistribution {
   private:
     boost::random::binomial_distribution<int> dist;