Change nonce to _wpnonce

Fixes #74

Updating to reflect
WP-API/WP-API#1524
https://core.trac.wordpress.org/ticket/34375

Author: Shelob9

guide/authentication/index.md

@@ -28,7 +28,7 @@ correctly for any custom requests.
 
 For developers making manual Ajax requests, the nonce will need to be passed
 with each request. The API uses nonces with the action set to `wp_rest`. These
-can then be passed to the API via the `_wp_rest_nonce` data parameter (either POST data or in the query for GET requests), or via the `X-WP-Nonce` header.
+can then be passed to the API via the `_wpnonce` data parameter (either POST data or in the query for GET requests), or via the `X-WP-Nonce` header.
 
 It is important to keep in mind that this authentication method relies on WordPress cookies. As a result this method is only applicable when the REST API is used inside of WordPress and the current user is logged in. In addition, the current user must have the appropriate capability to perform the action being performed.