Fix out-of-sync text and code in syscall implementation guide

[pjhades]

In the syscall implementation guide, the code example uses both which and command_num while the text uses which. This PR fixes this mismatch.

[lschuermann]

Thanks! There are a few other uses of which for command in other code blocks in this file, would you mind updating those as well for consistency's sake?

[pjhades]

Good catch! PR is updated.