fix Accept header typo

Author: dpassage

README.md

@@ -90,7 +90,7 @@ be specified with all requests. Default versions should be avoided as they are
 very difficult, at best, to change in the future.
 
 It is best to provide version specification in the headers, with other
-metadata, using the `Accepts header with a custom content type, e.g.:
+metadata, using the `Accept` header with a custom content type, e.g.:
 
 ```
 Accept: application/vnd.heroku+json; version=3