riscv-semihosting: fix ambiguous documentation link

[00xc]

Fix the following warning when generating the crate documentation:

warning: `syscall` is both a function and a macro
   --> riscv-semihosting/src/lib.rs:212:15
    |
212 | /// Same as [`syscall`].
    |               ^^^^^^^ ambiguous link
    |
    = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default
help: to link to the function, add parentheses
    |
212 | /// Same as [`syscall()`].
    |                      ++
help: to link to the macro, add an exclamation mark
    |
212 | /// Same as [`syscall!`].
    |                      +