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)