reverse-string: fix approach typo (#942)

Author: kossnikita

exercises/practice/reverse-string/.approaches/introduction.md

@@ -169,7 +169,7 @@ std::string reverse_string(std::string_view original)
     std::string result;
 
     // pass it to the recursive helper function
-    helper(original, result);
+    recursive_helper(original, result);
 
     // result now contains a reversed version of the string
     return result;

exercises/practice/reverse-string/.approaches/linear-recursion/content.md

@@ -42,7 +42,7 @@ void recursive_helper(std::string_view input, std::string& output)
 std::string reverse_string(std::string_view original)
 {
     std::string result;
-    helper(original, result);
+    recursive_helper(original, result);
     return result;
 }