Map interrupt 0 to IRQ 0

In trap.t, line 699 says:

> xv6 programs to map interrupt 0 to IRQ 0

This sentence is not particularly clear. I would suggest:

```
During initialization, xv6 programs interrupt gates:
.code-index tvinit
.line trap.c:/^tvinit/
initialize entry 0 of IDT to vector0, and
so on.
```