Fix error help message.

Before: 

```
$ ./doozer set
set: wrong number of arguments
Use: %!s(func()=0x402f19) [options] set [options] <path> <rev>
...
```

After:

```
$ ./doozer set
set: wrong number of arguments
Use: ./doozer [options] set [options] <path> <rev>
...
```