Fix full path being output with rustdoc -h

[mgattozzi]

rustdoc would output the full path to the binary when calling it with
the -h or --help flags. This is undesired behavior. It has been
replaced with a hardcoded string rustdoc to fix the issue.

Fixes#39310

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @steveklabnik (or someone else) soon.

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[mgattozzi]

@steveklabnik have you had a chance to look at this?

[alexcrichton]

@bors: r+

[bors]

📌 Commit bb34856 has been approved by alexcrichton

[bors]

⌛ Testing commit bb34856 with merge 9c8cdb2...