doc: Fix `arm-none-eabi-gdb` installation instruction for Fedora 27 or newer to just use `gdb`

[mavjs]

• Fixes rust-embedded/book#249
• In the Rust Embedded Discovery book, it is noted to use gdb instead of arm-none-eabi-gdb as pointed out in rust-embedded/discovery#364 that gdb on Fedora justworks(tm). This was fixed for Discovery book via rust-embedded/discovery#376
• Likewise, as mentioned per my comment in rust-embedded/book#249, on Fedora 37, gdb justworks(tm) as well.

[rust-highfive]

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

Please see the contribution instructions for more information.

[mavjs]

@eldruin Thanks for approving my pull request, however, it seems the checks via bors is pending. Is there anything I should do on my end?

[adamgreig]

bors retry

[bors[bot]]

Build succeeded:

• build