Update tier-migration.md

Please update these changes in Line 56.
Migration is only available and can be completed using the Azure portal. Service charges for Azure Front Door Standard or Premium tier will start once the migration is completed.

Author: asukumar22

articles/frontdoor/tier-migration.md

@@ -53,7 +53,7 @@ Azure Front Door zero-down time migration happens in three stages. The first sta
 > * Traffic to your Azure Front Door (classic) will continue to be serve until migration has been completed. 
 > * Each Azure Front Door (classic) profile can create one Azure Front Door Standard or Premium profile.
 
-Migration is only available can be completed using the Azure portal. Service charges for Azure Front Door Standard or Premium tier will start once migration is completed.
+Migration is only available and can be completed using the Azure portal. Service charges for Azure Front Door Standard or Premium tier will start once migration is completed.
 
 ## Breaking changes between tiers