Fixes nits in string guide

/cc @huonw 
Maybe you could write this with explicit types to make it clearer that the function is being called polymorphically.

```rust
let strs: &[&str] = &["Hello, World"];
println!("{}", sum_of_string_length(strs));

let strings: &[String] = &["Hello, world".to_string()];
println!("{}", sum_of_string_length(strings));
```