Add more context create/drop database examples

This is poorly written documentation. The variable dbContext is used out of context - missing the code that declares and initializes it.

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#### Document Details

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* ID: d1e258bc-5d33-0586-e2ba-46dbdf2e3d35
* Version Independent ID: 777c4acb-f5c7-3b99-950e-e5265006182a
* Content: [Create and Drop APIs - EF Core](https://docs.microsoft.com/en-us/ef/core/managing-schemas/ensure-created#feedback)
* Content Source: [entity-framework/core/managing-schemas/ensure-created.md](https://github.com/aspnet/EntityFramework.Docs/blob/master/entity-framework/core/managing-schemas/ensure-created.md)
* Product: **entity-framework**
* Technology: **entity-framework-core**
* GitHub Login: @bricelam
* Microsoft Alias: **bricelam**