Minor improvements to dependency injection example

[FrankRay78]

This PR is part of spectreconsole/spectre.console#1620 and should be merged at the same time.

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Changes:

• Updated code example to align with snippet in CommandApp/DI documentation
• Renamed TypeRegistrar to MyTypeRegistrar, making it more obvious it is a custom class the user must provide.

[welbertwpg]

I agree 100%. I got confused with the TypeRegistrar example and had to refer to the example code here to understand what I was missing, especially because the package also contains a class with the exact same name.