more descriptive documentation in example

I wasn't sure what that parameter was at first glance

README.md

@@ -147,6 +147,7 @@ You can also get a string representation (serialize):
 std::string s = j.dump();    // {\"happy\":true,\"pi\":3.141}
 
 // serialization with pretty printing
+// pass in the amount of spaces to indent
 std::cout << j.dump(4) << std::endl;
 // {
 //     "happy": true,