Match number of samples and math in novice shell Introduction

The number of samples is originally reported as 1520, but when
discussing how many possible combinations there are it is treated
as 300 samples. 1520 leads to some fairly crazy numbers for the
amount of runs involved so reduce the number of samples to 300
to match the existing calculations.
Merge pull request #410 from ethanwhite/math-fix

Match number of samples and math in novice shell Introduction