Font path fix

[jorgeolothar]

In latex2png.py -> zhtext2pil(), the font directory should be included:

fontText = ImageFont.truetype("simhei.ttf", 15, encoding="utf-8")

should be

fontText = ImageFont.truetype("assets/fonts/simhei.ttf", 15, encoding="utf-8")

[wufan-tb]

Thanks for your kindly correction, it is a careless mistake indeed and we are going to fix it in the new version.