Everything Is Just Functions: 1 week with David Beazley and SICP

There's a typo in the code in "the substitution model" section:

  ("+", ("fib", ("-", "n", 2)), ("fib", ("-", "n", 1))),

The two calls to `fib` are surely meant to be `fibonacci` since the latter is defined, but not the former. Indeed, the code is correct in the github repo:

https://github.com/savarin/pyscheme/blob/0f47292c8e5112425b5...