fix error in string-copy example of "Avoiding pitfalls"

Avoiding-pitfalls.md

@@ -84,7 +84,7 @@ For instance the following code will not work as expected:
     JsonArray& array = jsonBuffer.createArray();
     for (int i=0; i<3; i++) {
         char buffer[16];
-        sprintf(buffer, "iteration %d", 0);
+        sprintf(buffer, "iteration %d", i);
         array.add(buffer);
     }
     array.printTo(Serial);