use media-endpoint for consistency

lib/helpers.php

@@ -239,8 +239,8 @@ function get_micropub_config(&$user, $query=[]) {
     $user->syndication_targets = json_encode($targets);
 
   $media_endpoint = false;
-  if(array_key_exists('media_endpoint', $r['data'])) {
-    $media_endpoint = $r['data']['media_endpoint'];
+  if(array_key_exists('media-endpoint', $r['data'])) {
+    $media_endpoint = $r['data']['media-endpoint'];
     $user->micropub_media_endpoint = $media_endpoint;
   }