Fix variable typo

Author: jmikola

docs/examples/queryable_encryption-explicit.php

@@ -91,4 +91,4 @@
 
 /* Using the client configured with encryption (but not automatic encryption),
  * find the document and observe that the fields are automatically decrypted. */
-print_r($encryptedCollection->findOne(['encryptedIndexed' => $findPayload]));
+print_r($encryptedCollection->findOne(['encryptedIndexed' => $indexedFindPayload]));