Fix missing comma in options example (#145)

Author: fcFn

README.md

@@ -152,7 +152,7 @@ for plain browser:
     mode: 'cors',
     credentials: 'same-origin',
     cache: 'default'
-  }
+  },
 
   // define a custom request function
   // can be used to support XDomainRequest in IE 8 and 9