concat: Fix active-pad property doc typo

Part-of: <!727>

docs/plugins/gst_plugins_cache.json

@@ -135,7 +135,7 @@
                 },
                 "properties": {
                     "active-pad": {
-                        "blurb": "Currently active src pad",
+                        "blurb": "Currently active sink pad",
                         "conditionally-available": false,
                         "construct": false,
                         "construct-only": false,

plugins/elements/gstconcat.c

@@ -165,7 +165,7 @@ gst_concat_class_init (GstConcatClass * klass)
   gobject_class->set_property = gst_concat_set_property;
 
   pspec_active_pad = g_param_spec_object ("active-pad", "Active pad",
-      "Currently active src pad", GST_TYPE_PAD, G_PARAM_READABLE |
+      "Currently active sink pad", GST_TYPE_PAD, G_PARAM_READABLE |
       G_PARAM_STATIC_STRINGS);
   g_object_class_install_property (gobject_class, PROP_ACTIVE_PAD,
       pspec_active_pad);