fix a typo error in fibo_speed.rst

[Iamjack]

def fib_cached(n, cache={}): if n < 2: return n try: val = cache[n] except KeyError: val = fib(n-2) + fib(n-1) cache[n] = val return val

Maybe there is a typo error in function body : val = fib(n-2) + fib(n-1) should be like this: val = fib_cached(n-2,cache) + fib_cached(n-1,cache)