Replace one last expect-error with an as

In this case the 'as' is asserting "I know better than the compiler" and it
is both true and more accurate than saying "I expect typescript to error out
here." As an added benefit, `as` does more type checking than 'expect error',
so you get just a small bit more type safety.

src/Router.ts

@@ -59,7 +59,6 @@ export const Router = <
   RequestType = IRequest,
   Args extends any[] = any[]
 >({ base = '', routes = [] }: RouterOptions = {}): RouterType<RequestType, Args> =>
-  // @ts-expect-error TypeScript doesn't know that Proxy makes this work
   ({
     __proto__: new Proxy({}, {
       // @ts-expect-error (we're adding an expected prop "path" to the get)
@@ -97,4 +96,4 @@ export const Router = <
         }
       }
     }
-  })
+  } as RouterType<RequestType, Args>)