Update articles/firewall/easy-upgrade.md

Added period to alt text description.

Author: v-stsavell

articles/firewall/easy-upgrade.md

@@ -26,7 +26,7 @@ In the upgrade process, you can select the policy to be attached to the upgraded
 
 This new capability is available through the Azure portal as shown here. It's also available via PowerShell and Terraform by changing the sku_tier attribute.
 
-:::image type="content" source="media/premium-features/upgrade.png" alt-text="Screenshot showing SKU upgrade" lightbox="media/premium-features/upgrade.png":::
+:::image type="content" source="media/premium-features/upgrade.png" alt-text="Screenshot showing SKU upgrade." lightbox="media/premium-features/upgrade.png":::
 
 > [!NOTE]
 > This new upgrade/downgrade capability will also support the Basic SKU for GA.