Base "middle" on 0-index consistently

This section of the lesson is inconsistent. It uses data[30,20] as a midpoint but states that this is the 30th row and 20th column.

Either the indices should be changed (per PR) and the text updated a little or the text should be changed to reflect that this is the 31st row and 21st column (not 30th and 20th).

Author: ndporter

_episodes/02-numpy.md

@@ -205,16 +205,16 @@ first value in data: 0.0
 {: .output}
 
 ~~~
-print('middle value in data:', data[30, 20])
+print('middle value in data:', data[29, 19])
 ~~~
 {: .language-python}
 
 ~~~
-middle value in data: 13.0
+middle value in data: 16.0
 ~~~
 {: .output}
 
-The expression `data[30, 20]` accesses the element at row 30, column 20. While this expression may
+The expression `data[29, 19]` accesses the element at row 30, column 20. While this expression may
 not surprise you,
  `data[0, 0]` might.
 Programming languages like Fortran, MATLAB and R start counting at 1