fixed: use github2.js instead of the case of typo for github23.js

In week4, we made typo error, github23.js instead of github2.js.
However, many students did not modify the link at README.md,
so just use [github2.js] instead of real link (github23.js).

However, reality is that check any .js or .json result for
grading by each answer by student.