minor typo

[christian-simmons]

In Lesson 3

Line 339 "Then to answer the above question on the mean flow during 1951-1971,..." should be "1951-1970" because the Nile set only goes that far. The actual code is correct though.

Edit:
There are actually a few more. I will list them below as I read through the book and update this:

Line 535 "arbirary, just something to help me remember what is stored in that"
Line 598 "**Note that that row would also be cnsidered a vector. The third column"
Line 1068 "say below 500 or above 1200:" should be "say below 800 or above 1300:" to match code example

Enjoying so far as a refresh from college. Thank you.