add missing backticks (oops)
//!
//! A nested subcommand can be marked optional:
//!
+//! ```ignore
//! #[derive(StructOpt)]
//! #[structopt(name = "foo")]
//! struct Foo {
//! Baz {},
//! Quux {}
//! }
+//! ```
extern crate proc_macro;
extern crate syn;
