[guide] Use `acc` as `reduce` accumulator instead of `memo` to make it valid
[guide] Use `acc` as `reduce` accumulator instead of `memo` to make it valid

In [this part of the `no-param-reassign` configuration](https://github.com/airbnb/javascript/blob/53b2d7d245ba4abefc0429bfda4a46f099b9ace5/packages/eslint-config-airbnb-base/rules/best-practices.js#L174-L175), `acc` is allowed to be mutated (rule [7.12](https://github.com/airbnb/javascript#functions--mutate-params), but `memo` is not.

This causes the [rule 4.6 example](https://github.com/airbnb/javascript#arrays--callback-return) in the README to actually be invalid.

airbnb/javascript,cloudability/javascript-style-guide,ljharb/javascript,bl00mber/javascript,getjll/JavaScript-Style-Guide