Compiler warning in src/main.rs

## Generic Issue

- [x] Ensure this issue is not a bug report or feature request

**Description**
The following appears using rustc 1.58.1:
```
% cargo check
warning: derive helper attribute is used before it is introduced
--> src/main.rs:9:3
|
9 | #[structopt(
| ^^^^^^^^^
...
13 | #[derive(StructOpt)]
| --------- the attribute is introduced here
|
= note: `#[warn(legacy_derive_helpers)]` on by default
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #79202

warning: `electionguard_verify` (bin "verify") generated 1 warning
Finished dev [unoptimized + debuginfo] target(s) in 0.05s
```

**Possible Solution**

**Additional context/Screenshots**
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