chore: fix a minor typo

exercises/09_strings/strings3.rs

@@ -11,7 +11,7 @@ fn trim_me(input: &str) -> String {
 }
 
 fn compose_me(input: &str) -> String {
-    // TODO: Add " world!" to the string! There's multiple ways to do this!
+    // TODO: Add " world!" to the string! There are multiple ways to do this!
     ???
 }