docs: bugfix example and make idiomatic to express

[coolaj86]

I'm assuming that this example is intended to be geared towards people who use express, so I made it more express-like.

• fix use of single quotes where template string was intended
• add "use strict" because this is node code, not transpiled
• use app (the normal name of an express router) rather than server
• no redundant return which may be confusing to the reader (there's nothing being returned)
• consistent use of single quotes (rather than mixing single and double quotes)
• go full-on async/await (using .catch() rather than try{})
• use express' built-in next(err) handler rather than a one-off error handler
• add some whitespace for readability

Preview

See the concise diff at 46a5302 (this is built in part on the prettier PR, but just the README part)

Preview at https://github.com/coolaj86/su-sdk/tree/express#execution