Update example/getHeader

example/getHeader/1.0/policy.rego

@@ -2,4 +2,4 @@ package example.getHeader
 
 # gets the value for "x-cache-key" header and return it in the result
 # result is empty if the "x-cache-key" header is not sent in the request
-key = input.headers["X-Cache-Key"]
+key := external.http.header("X-Cache-Key")