Modify code to read files in directory in alphabetical order

[seungwoochoe]

[seungwoochoe]

Oops, I was going to make pull request in a forked repository..

I'm using macOS Big Sur 11.6 and gem 3.0.3 (I've never used Ruby before and I'm not sure if this is all information you needed). I used this program to create one pdf file from multiple source codes in one directory, but the order of files was totally random (3, 9, 7, 4, 1, and so on...).

This commit makes code2pdf to read files in directory in alphabetical order.