Update chapter-11.md
Update chapter-11.md

Fixes

```error: format argument must be a string literal.
stdin.rs:10   println!(input);
                       ^~~~~
error: aborting due to previous error
```
steveklabnik/rust_for_rubyists,gpbonillas/rust_for_rubyists,gpbonillas/rust_for_rubyists,gpbonillas/rust_for_rubyists,steveklabnik/rust_for_rubyists,dskecse/rust_for_rubyists,dskecse/rust_for_rubyists,steveklabnik/rust_for_rubyists,dskecse/rust_for_rubyists