Make example in interactive shell copy-pastable.

Closes numpygh-240

Author: rgommers

layouts/partials/shell-lesson.html

@@ -1,14 +1,10 @@
-
-The standard way to import NumPy:
-
 ```python
+>>> # The standard way to import NumPy:
 >>> import numpy as np
-```
 
-Create a 2-D array, set every second element in some rows, and find max per
-row:
+>>> # Create a 2-D array, set every second element in
+>>> # some rows and find max per row:
 
-```python
 >>> x = np.arange(15, dtype=np.int64).reshape(3, 5)
 >>> x[1:, ::2] = -99
 >>> x
@@ -17,10 +13,8 @@
        [-99,  11, -99,  13, -99]])
 >>> x.max(axis=1)
 array([ 4,  8, 13])
-```
 
-Generate some random numbers with an exponential distribution:
-```python
+>>> # Generate normally distributed random numbers:
 >>> rng = np.random.default_rng()
->>> samples = rng.exponential(size=2500)
+>>> samples = rng.normal(size=2500)
 ```