Rename first_word to first_letter

solutions/compound-types/slice.md

@@ -62,16 +62,16 @@ fn main() {
 fn main() {
     let mut s = String::from("hello world");
 
-    // here, &s is `&String` type, but `first_word` need a `&str` type.
+    // here, &s is `&String` type, but `first_letter` needs a `&str` type.
     // it works because `&String` can be implicitly converted to `&str, If you want know more ,this is called `Deref` 
-    let word = first_word(&s);
+    let letter = first_letter(&s);
 
-    println!("the first word is: {}", word);
+    println!("the first letter is: {}", letter);
 
     s.clear();
 }
 
-fn first_word(s: &str) -> &str {
+fn first_letter(s: &str) -> &str {
     &s[..1]
 }
 ```