Configuration reference: Improve example of http_client::headers

Author: Nyholm

reference/configuration/framework.rst

@@ -873,7 +873,7 @@ headers
 **type**: ``array``
 
 An associative array of the HTTP headers added before making the request. This
-value must use the format ``['header-name' => header-value, ...]``.
+value must use the format ``['header-name' => 'value0, value1, ...']``.
 
 http_version
 ............