Fixes syntax error in option_spec

Removes extra closing parenthesis, revealing a legitimate test failure
```
Failures:

  1) Thor::Option#usage uses padding when no aliases are given
     Failure/Error: expect(parse(:foo, :boolean).usage).to eq("    [--foo]")
       
       expected: "    [--foo]"
            got: "[--foo]"
       
       (compared using ==)
```