Fix lovr.headset.update return;

api/init.lua

@@ -24346,7 +24346,13 @@ return {
           variants = {
             {
               arguments = {},
-              returns = {}
+              returns = {
+                {
+                  name = "dt",
+                  type = "number",
+                  description = "The delta time since the last frame.  This is the same value returned by `lovr.headset.getDeltaTime`, and is used by boot.lua."
+                }
+              }
             }
           }
         },

api/lovr/headset/update.lua

@@ -19,7 +19,7 @@ return {
   variants = {
     {
       arguments = {},
-      returns = {}
+      returns = { 'dt' }
     }
   },
   related = {