Add example of non-mutable mapping to cheat sheet

[MarcoGorelli]

closes #7617

[msullivan]

Thinking about this a bit, this example is correct but I think fails to capture the main reason you usually want to use Mapping over MutableMapping: to ensure that even if the mapping happens to be mutable (which it usually is, since it is usually just a dict), that the function doesn't mutate it.

This is useful in two ways:

1. Just in a general correctness/reasoning sense. Making it immutable means the caller can't mutate it, which makes reasoning easier and might prevent someone introducing a later change that mutates it.
2. From a type system perspective, Mapping is covariant in its second argument, which means that you can, for example, pass a Dict[int, str] to a function expecting a Mapping[int, Optional[str]], for example. (Which you can't do if the function took a Dict or a MutableMapping)

[MarcoGorelli]

Ah, I understand - so the difference isn't in the object you pass, but in what you do with that object. I've tried updating the PR so this is more explicit, but perhaps it's clear enough as is, in which case, feel free to close this :)

[gvanrossum]

Nice!