README: Corrected confusing example (#98)

`upstream` is passed as `base` to `fastify-reply-from`. Quoting `fastify-reply-form` doc:

> *base*
> Set the base URL for all the forwarded requests. Will be required if http2 is set to true Note that path will be discarded.

So, specifying `/auth` in `upstream` has no effect at all, but this example seems to suggest `/auth/user` requests will be forwarded to `http://single-signon.example.com/auth/user`, when the real target will be `http://single-signon.example.com/user`.

Author: nahuel

README.md

@@ -46,15 +46,18 @@ const Fastify = require('fastify')
 const server = Fastify()
 const proxy = require('fastify-http-proxy')
 
+// /api/x will be proxied to http://my-api.example.com/x
 server.register(proxy, {
   upstream: 'http://my-api.example.com',
   prefix: '/api', // optional
   http2: false // optional
 })
 
+// /auth/user will be proxied to http://single-signon.example.com/signon/user
 server.register(proxy, {
-  upstream: 'http://single-signon.example.com/auth',
+  upstream: 'http://single-signon.example.com',
   prefix: '/auth', // optional
+  rewritePrefix: '/signon', // optional
   http2: false // optional
 })
 
@@ -63,6 +66,8 @@ server.listen(3000)
 
 Notice that in this case it is important to use the `prefix` option to tell the proxy how to properly route the requests across different upstreams.
 
+Also notice paths in `upstream` are ignored, so you need to use `rewritePrefix` to specify the target base path.
+
 For other examples, see `example.js`.
 
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