n80100 changed to n81100 for consistency

[chatasma]

The assignment operator example copies the 81st to 100th elements of the Nile dataset to the variable n81100, but the subsequent explanation refers to it as n80100. I changed the subsequent explanation to use n81100 instead of n80100 (and made other appropriate changes) to stay consistent with the example.

[matloff]

Thanks very much, chatasma! I've made the fixes.