Typo

src/core/database/json/mod.rs

@@ -151,7 +151,7 @@ mod tests {
 
     #[test]
     fn save_errors() {
-        // serde_json will raise an error as it requires keys to be a string.
+        // serde_json will raise an error as it requires keys to be strings.
         let mut object = HashMap::<Option<String>, String>::new();
         object.insert(Some(String::from("artist")), String::from("string"));
         let serde_err = serde_json::to_string(&object);