page 141, incorrect indentation, countdown() middle of page

Page 141, `countdown` definition in the middle of the page.

The `try` block has the following code:
```
while n > 0:
    yield n
        n = n - 1
```
The `n = n - 1` line has extra indentation and should align with the `yield` line.
It generates the following error:
```
...         n = n - 1
  File "<stdin>", line 6
    n = n - 1
IndentationError: unexpected indent
```