📝 fixed examples for Wandbox

As I learned in https://github.com/melpon/wandbox/issues/209, this
library is already installed at Wandbox, so we need to adjust the
examples to use `#include "json.hpp"` insteas of `#include <json.hpp>`.

doc/examples/operator__notequal.output

@@ -1,4 +1,4 @@
 [1,2,3] == [1,2,4] true
 {"A":"a","B":"b"} == {"A":"a","B":"b"} false
-17 == 17 false
+17 == 17.0 false
 "foo" == "bar" true