add correct Accept header to graphql CORS example

by default the browser sends */* which hits the error pages, may disable these
by default in the future though since I dont think anyone uses Accept these days haha

Author: tj

oss/node-graphql/Readme.md

@@ -35,7 +35,7 @@ const body = JSON.stringify({
 })
 
 const res = await fetch('http://localhost:3000', {
-  headers: { 'Content-Type': 'application/json' },
+  headers: { 'Content-Type': 'application/json', Accept: 'application/json' },
   method: 'POST',
   body
 })