Add the with keyword when opening files.

[kotaroyama]

This is a really minor change to modity.py. According to the official Python documentation, it's always a good idea to use the "with" keyword when opening files. (Refer to Section 7.2 in the page.) Hence, I have modified modify.py so that it opens all the files with the "with" keyword. (Namely "JSON_ERROR_LIST.txt".)