Update solution files to match exercise files

solutions/09_strings/strings4.rs

@@ -19,7 +19,7 @@ fn main() {
     // `.into()` converts a type into an expected type.
     // If it is called where `String` is expected, it will convert `&str` to `String`.
     string("nice weather".into());
-    // But if it is called where `&str` is expected, then `&str` is kept `&str` since no conversion is needed.
+    // But if it is called where `&str` is expected, then `&str` is kept as `&str` since no conversion is needed.
     // If you remove the `#[allow(…)]` line, then Clippy will tell you to remove `.into()` below since it is a useless conversion.
     #[allow(clippy::useless_conversion)]
     string_slice("nice weather".into());