Clarify shebang requirement for running C# files directly on Unix systems.

[shethaadit]

Summary

Clarifies that C# files need a shebang (#!) directive to be directly executable on Unix systems with ./filename.cs.

Changes:

• Added "(shebang)" after #! for clarity
• Modified sentence to specify "such a C# file that contains the shebang directive"
• Added "directly" to distinguish from dotnet run

Before:

On any unix system, if you set the execute (+x) permission on a C# file, you can run the C# file from the command line:

After:

On any unix system, if you set the execute (+x) permission on such a C# file that contains the shebang directive, you can run the C# file directly from the command line:

This prevents beginner confusion about why ./hello.cs doesn't work without the shebang.

Fixes #47108

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Internal previews

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docs/csharp/tour-of-csharp/overview.md	A tour of the C# language

[BillWagner]

Thanks again @shethaadit

This looks great. Let's