fix: replace c++ with cpp for language postfix

guide/english/cplusplus/input-and-output/index.md

@@ -11,7 +11,7 @@ To print things to the console, or read from it, you use `cout` and `cin`, which
 The "Hello World" program uses `cout` to print "Hello World!" to the console:
 
 ```cpp
-#include<iostream.h>
+#include<iostream>
 using namespace std; 
 
 int main()