Fix response body example.

The initial promise is resolved when the response headers are
available, not necessarily when the entire body has been received. The
body must be processed by resolving another promise (text, json, blob).

README.md

@@ -26,9 +26,12 @@ example requests.
 ### HTML
 
 ```javascript
-fetch('/users.html').then(function(response) {
-  document.body.innerHTML = response.body
-})
+fetch('/users.html')
+  .then(function(response) {
+    return response.text()
+  }).then(function(body) {
+    document.body.innerHTML = body
+  })
 ```
 
 ### JSON
@@ -52,7 +55,6 @@ fetch('/users.json').then(function(response) {
   console.log(response.headers.get('Date'))
   console.log(response.status)
   console.log(response.statusText)
-  console.log(response.body)
 })
 ```