[keyvault-secrets] Change incorrectly named expires property to expiresOn in README (Azure#26507)

### Packages impacted by this PR

- `@azure/keyvault-secrets`

### Issues associated with this PR

- Fixes Azure#26431

### Describe the problem that is addressed by this PR

Documentation fix as reported in Azure#26431.

Author: timovv

sdk/keyvault/keyvault-secrets/README.md

@@ -205,7 +205,7 @@ the following attributes:
 - `contentType`: Any string that can be used to help the receiver of the secret understand how to use the secret value.
 - `enabled`: A boolean value that determines whether the secret value can be read or not.
 - `notBefore`: A given date after which the secret value can be retrieved.
-- `expires`: A given date after which the secret value cannot be retrieved.
+- `expiresOn`: A given date after which the secret value cannot be retrieved.
 
 An object with these attributes can be sent as the third parameter of
 `setSecret`, right after the secret's name and value, as follows: