Update introduction.md - change .append() to push_back() (#1044)

Changed result.append(original[rindex]) to result.push_back(original[rindex]) in Approach:Iterative code.

.append() does not compile on Exercism, or on Ubuntu using gcc or clang. Appears to be because append() does not work with empty strings.

Author: wintermute-alias

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

@@ -29,7 +29,7 @@ std::string reverse_string(const std::string& original)
         std::size_t rindex = size(original) - i - 1;
 
         // append each character (starting with the last one)
-        result.append(original[rindex]);
+        result.push_back(original[rindex]);
     }
 
     // result now contains a reversed version of the string