string slicing: enhance API to accept string slices

This way, the API is more general and we can accept any kind of string

string_slicing/src/main.rs

@@ -1,4 +1,4 @@
-fn first_word(s: &String) -> &str {
+fn first_word(s: &str) -> &str {
     let bytes = s.as_bytes();
 
     for (i, &item) in bytes.iter().enumerate() {