Update README.md with corrected S3 initialization

No need to take my word for it, you figured it out first :-)
https://medium.com/@maticzav/graphcool-1-0-examples-series-file-api-3b16b4b8785f

README.md

@@ -33,7 +33,7 @@ import { upload } from 'graphql-middleware-apollo-upload-server'
 const client = new S3({
   accessKeyId: __S3_KEY__,
   secretAccessKey: __S3_SECRET__,
-  Bucket: __S3_BUCKET__,
+  params: { Bucket: __S3_BUCKET__ },
 })
 
 const uploadToS3 = async file => {