Fix typo in Fibonacci example

[varunvats]

The line for computing fib_14 in the code snippet uses fib_13 twice:

>>> fib_14 = fib_13 + fib_13
>>> fib_14
377

While the result shown for fib_14 is correct (377), going by the above, the result of fib_14 would be 466 instead.

This line should instead be:

>>> fib_14 = fib_13 + fib_12