Update solution

Author: aruseni

1-js/05-data-types/03-string/1-ucfirst/solution.md

@@ -6,7 +6,7 @@ But we can make a new string based on the existing one, with the uppercased firs
 let newStr = str[0].toUpperCase() + str.slice(1);
 ```
 
-There's a small problem though. If `str` is empty, then `str[0]` is undefined, so we'll get an error.
+There's a small problem though. If `str` is empty, then `str[0]` is `undefined`, and as `undefined` doesn't have the `toUpperCase()` method, we'll get an error.
 
 There are two variants here: