Update 2DMOT15 references to MOT15 and fix broken link

[yugyeom-ghim]

While reviewing the documentation, I noticed references to the "2DMOT15" dataset. I've come to understand that the official name of the dataset has recently been changed from "2DMOT15" to "MOT15".

However, some parts of the documentation still refer to it as "2DMOT15". Given the previous experiments and references conducted with "2DMOT15", changing the name throughout might introduce confusion. Therefore, instead of altering the dataset name in the documentation, it seems more prudent to just update the broken link to prevent any confusion.

Proposed changes:

• Only fix the broken link that points to the 2DMOT15 dataset.

P.S. I plan to work on this issue and submit a PR soon.