readme: fix various typos

L6: remove redundant "writing".
L99: pluralize "middleware" as "middlewares" (see  [Wiktionary](https://en.wiktionary.org/wiki/middleware)).
L99: capitalize "facebook" as "Facebook".
LL77-95: pluralize "middleware" as "middlewares".

Because discrimination seems to be an issue lately and we don't want Facebook to feel insulted. :wink: