docs: Fix definition of middleware in guide.md (#1810)

docs/guide.md

@@ -18,7 +18,7 @@
 
 ## Writing Middleware
 
-  Koa middleware are simple functions which return a `MiddlewareFunction` with signature (ctx, next). When
+  Koa middleware are simple functions with the signature `(ctx, next)` (i.e. a `MiddlewareFunction`). When
   the middleware is run, it must manually invoke `next()` to run the "downstream" middleware.
 
   For example if you wanted to track how long it takes for a request to propagate through Koa by adding an