Fix example class name in docstrings

Replaces incorrect 'VeA2ACredentialService' with 'VeCredentialService' in usage examples within docstrings to ensure accuracy and prevent confusion for users.

Author: loveyana

veadk/auth/credential_service.py

@@ -147,7 +147,7 @@ async def set_credential(
             ```python
             from google.adk.auth.auth_credential import AuthCredential, AuthCredentialTypes
 
-            service = VeA2ACredentialService()
+            service = VeCredentialService()
             await service.set_credential(
                 app_name="my_app",
                 user_id="user123",
@@ -190,7 +190,7 @@ async def get_credential(
 
         Examples:
             ```python
-            service = VeA2ACredentialService()
+            service = VeCredentialService()
             credential = await service.get_credential(
                 app_name="my_app",
                 user_id="user123",