Switch example to SECONDARY_PREFERRED

Author: GromNaN

docs/includes/query-builder/QueryBuilderTest.php

@@ -457,7 +457,7 @@ public function testReadPreference(): void
         // begin query read pref
         $result = DB::table('movies')
             ->where('runtime', '>', 240)
-            ->readPreference('secondary')
+            ->readPreference(\MongoDB\Driver\ReadPreference::SECONDARY_PREFERRED)
             ->get();
         // end query read pref