Fix error help message.

Before: 

```
$ ./doozer set
set: wrong number of arguments
Use: %!s(func()=0x402f19) [options] set [options] <path> <rev>

Sets the body of the file at <path>.

The body is read from stdin. If <rev> is not greater than or equal to
the revision of the file, no change will be made.

Prints the new revision on stdout, or an error message on stderr.
```

After:

```
$ ./doozer set
set: wrong number of arguments
Use: ./doozer [options] set [options] <path> <rev>

Sets the body of the file at <path>.

The body is read from stdin. If <rev> is not greater than or equal to
the revision of the file, no change will be made.

Prints the new revision on stdout, or an error message on stderr.
```