Fix CSS in Iframe for whatsapp integration (#3599)

Summary:
## Description
Fix CSS in iframe for whatsapp integration

### Type of change
- Fix (non-breaking change which fixes an issue)

## Changelog entry
Fix CSS in Iframe for whatsapp integration

Pull Request resolved: #3599

Test Plan:
### Before
<img width="1728" height="962" alt="Screenshot 2025-09-11 at 5 51 39 PM" src="https://github.com/user-attachments/assets/26f7662c-292d-4a6b-b2c9-1d02328221ad" />

### After
<img width="1728" height="958" alt="Screenshot 2025-09-11 at 5 51 08 PM" src="https://github.com/user-attachments/assets/6a97a7be-d85a-4a9c-b2db-e855a83cf00a" />

Reviewed By: woo-ardsouza

Differential Revision: D82273097

Pulled By: sharunaanandraj

fbshipit-source-id: 9fdbfae12a901548b58a7ac4d2068e797e81abba

Author: sharunaanandraj

assets/css/admin/facebook-for-woocommerce-whatsapp-enhanced.css

@@ -1,5 +1,4 @@
- .facebook-whatsapp-iframe-container {
-    display: flex;
+.facebook-whatsapp-iframe-container {
     justify-content: center;
     margin: 0 auto;
  }