Add subsections in Section 3 for a better structure

[AndreG-P]

Reviewer 1 said:

Section 3 gives a good summarization of the possible problems during the translation, including undefined function, alteration and branch cuts, insufficient semantic label, ambiguity for the tokenization and structure analysis. However, it would be easier for reader to quickly capture these ideas if they are organized in subsections.

The official author guidelines said about headings:

Headings must be concise, with a clear indication of the distinction between the hierarchy of headings. The preferred format is for first level headings to be presented in bold format and subsequent sub-headings to be presented in medium italics.