[PATCH] doc: qemu-riscv: Fix typo and add network testing section

[Manjae Cho]

Hi Heinrich,

Thank you for the feedback and suggestion!

I agree that network testing documentation would be more useful as a 
separate file. I'll prepare v2 with the following changes:

- Patch 1/2: Fix the typo only
- Patch 2/2: Add doc/board/emulation/network.rst with QEMU network 
  testing examples

This approach will make it easier to expand the documentation with 
additional network configurations (e1000, tap/tun, MAC settings, etc.) 
in the future.

Best regards,
Manjae Cho