Fix: Correct Markdown Formatting for File Name in Dependency Injection Basics Doc

[HEJOK254]

Summary

Seems like there is a missing underscore on line 64 in the file name, which breaks the formatting:

Then add a new C# file named DefaultGreetingService.cs_ and add the following code:

changed to:

Then add a new C# file named _DefaultGreetingService.cs_ and add the following code:

---

Internal previews

📄 File	🔗 Preview link
docs/core/extensions/dependency-injection-basics.md	docs/core/extensions/dependency-injection-basics

[BillWagner]

Thanks @HEJOK254

This looks great, and I'll now.