Bug in user/src/bin/sig_simple2.rs

The 2nd parameter of `kill` [here](https://github.com/rcore-os/rCore-Tutorial-v3/blob/ch7/user/src/bin/sig_simple2.rs#L32) should be `10`, not `1<<10`. Otherwise you'll get `1<<(1<<10)` in [os/src/syscall/process.rs#sys_kill](https://github.com/rcore-os/rCore-Tutorial-v3/blob/ch7/os/src/syscall/process.rs#L109)