Update product_docs/docs/pgd/5.7/planning/limitations.mdx

Author: djw-m

product_docs/docs/pgd/5.7/planning/limitations.mdx

@@ -100,7 +100,7 @@ Therefore, once an PGD cluster upgrade has begun, you should complete the whole
 cluster upgrade as quickly as possible.
 
 We don't support running mixed versions of PGD except during an upgrade, and we
-don't support clusters running mixed versions even while being upgraded, for a
+don't support clusters running mixed versions even while being upgraded, for 
 extended periods.
 
 For more information on rolling upgrades and mixed versions, see [Rolling upgrade considerations](/pgd/latest/upgrades/manual_overview#rolling-upgrade-considerations).