Run Doctest wrong command line

Current command line in VS Code:

```
Executing task: cargo test --doc --package rhai -- scope:Scope:::get_mut --nocapture
                                                        ^     ^
```

Should be:

`Executing task: cargo test --doc --package rhai -- scope::Scope::get_mut --nocapture`

The `:` colon is at the wrong place.
