doc: fix type of http.request's agent option

PR-URL: #9584
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/http.md

@@ -1529,7 +1529,7 @@ added: v0.3.6
  * `headers` {Object} An object containing request headers.
  * `auth` {String} Basic authentication i.e. `'user:password'` to compute an
  Authorization header.
- * `agent` {String} Controls [`Agent`][] behavior. When an Agent is used request will
+ * `agent` {http.Agent|Boolean} Controls [`Agent`][] behavior. When an Agent is used request will
  default to `Connection: keep-alive`. Possible values:
  * `undefined` (default): use [`http.globalAgent`][] for this host and port.
  * `Agent` object: explicitly use the passed in `Agent`.