RISC-V: The riscv path of the debian sysroot had been changed

The url `https://httpredir.debian.org/debian-ports/` is not available for riscv after 2024-01-01. When using this url, the following error messages are outputted.

```
I: Retrieving InRelease
E: Invalid Release file, no entry for main/binary-riscv64/Packages
```

The riscv path is moved from `debian-ports` to the unified path `debian` at 2024-01-01, which is the same as arm.

So the building document should be adjusted. In detail, the following statement in `building.md` should be removed.

```
// building.md
If the target architecture is riscv64, the path should be debian-ports instead of debian.
```