Fix return type to match the interface

Author: GromNaN

src/PersistentCollection/PersistentCollectionTrait.php

@@ -8,6 +8,7 @@
 use Closure;
 use Doctrine\Common\Collections\Collection as BaseCollection;
 use Doctrine\Common\Collections\Criteria;
+use Doctrine\Common\Collections\ReadableCollection;
 use Doctrine\Common\Collections\Selectable;
 use Doctrine\ODM\MongoDB\DocumentManager;
 use Doctrine\ODM\MongoDB\Mapping\ClassMetadata;
@@ -779,7 +780,7 @@ public function reduce(Closure $func, $initial = null)
         return $this->coll->reduce($func, $initial);
     }
 
-    public function matching(Criteria $criteria)
+    public function matching(Criteria $criteria): ReadableCollection
     {
         $this->initialize();