Revisit the Overview of Entity Framework Core page

`optionsBuilder.UseSqlServer` won't work without adding reference to Microsoft.EntityFrameworkCore.SqlServer. You'll get the "...does not have extension method..." error by intellisense.

Tested with
* Microsoft.EntityFrameworkCore v3.1.8
* Microsoft.EntityFrameworkCore.SqlServer v3.1.8.

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: 1b15cd4c-9cba-c0e2-7618-7d621a3c5936
* Version Independent ID: f4681bc6-57be-6e69-cad9-776ce64d93bd
* Content: [Overview of Entity Framework Core - EF Core](https://docs.microsoft.com/en-us/ef/core/)
* Content Source: [entity-framework/core/index.md](https://github.com/dotnet/EntityFramework.Docs/blob/master/entity-framework/core/index.md)
* Product: **entity-framework**
* Technology: **entity-framework-core**
* GitHub Login: @ajcvickers
* Microsoft Alias: **avickers**