riscv: sigcontext: Correct the comment of sigreturn

The real-time signals enlarged the sigset_t type, and most architectures
have changed to using rt_sigreturn as the only way. The riscv is one of
them, and there is no sys_sigreturn in it. Only some old architecture
preserved sys_sigreturn as part of the historical burden.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20230628091213.2908149-1-guoren@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/uapi/asm/sigcontext.h

@@ -25,7 +25,7 @@ struct __sc_riscv_v_state {
  * Signal context structure
  *
  * This contains the context saved before a signal handler is invoked;
- * it is restored by sys_sigreturn / sys_rt_sigreturn.
+ * it is restored by sys_rt_sigreturn.
  */
 struct sigcontext {
 	struct user_regs_struct sc_regs;