Fix grammar: add comma for clarity in data-seeding.md

Agent-Logs-Url: https://github.com/dotnet/EntityFramework.Docs/sessions/f4ab7951-f2ec-45b3-867e-ea829f5aca95

Co-authored-by: roji <1862641+roji@users.noreply.github.com>

Author: Copilot

entity-framework/core/modeling/data-seeding.md

@@ -66,7 +66,7 @@ As an example, this will configure managed data for a `Country` in `OnModelCreat
 
 [!code-csharp[CountrySeed](../../../samples/core/Modeling/DataSeeding/ManagingDataContext.cs?name=CountrySeed)]
 
-To add entities that have a relationship the foreign key values need to be specified:
+To add entities that have a relationship, the foreign key values need to be specified:
 
 [!code-csharp[CitySeed](../../../samples/core/Modeling/DataSeeding/ManagingDataContext.cs?name=CitySeed)]