fix(readme): properly structure the options for custom agent (#786)

`request:` is nested under `options:` in the readme but in reality it's
passed at the same level as `username:` and `headers:`.

Resolves #785

Author: packrat386

README.md

@@ -566,10 +566,8 @@ const { data } = await request("GET /users/{username}", {
   headers: {
     "X-GitHub-Api-Version": "2022-11-28",
   },
-  options: {
-    request: {
-      fetch: myFetch,
-    },
+  request: {
+    fetch: myFetch,
   },
 });
 ```