set .cpp in readme

README.md

@@ -185,7 +185,7 @@ env.render("Content: {% include \"content\" %}", data); // "Content: Hello Peter
 render("{% include \"footer.html\" %}", data);
 ```
 If a corresponding template could not be found in the file system, the *include callback* is called:
-```
+```.cpp
 // The callback takes the current path and the wanted include name and returns a template
 env.set_include_callback([&env](const std::string& path, const std::string& name) {
   return env.parse("Hello {{ name }} from " + name);