fix declaration of expiresAt (#874)

Co-authored-by: Evan Sims <[email protected]>

Author: pnu

src/Cache/WpObjectCacheItem.php

@@ -42,7 +42,7 @@ public function set(mixed $value): static
         return $this;
     }
 
-    public function expiresAt(?\DateTimeInterface $expiration)
+    public function expiresAt(?\DateTimeInterface $expiration): static
     {
         if ($expiration instanceof DateTimeInterface) {
             $this->expires = $expiration->getTimestamp();