back-fill arXiv ID 2604.21668

humanml3d/README.md

@@ -99,7 +99,7 @@ does not supersede or relax the upstream license.
 @article{zhang2026smd,
   title   = {Encoder-Free Human Motion Understanding via Structured Motion Descriptions},
   author  = {Zhang, Yao and Liu, Zhuchenyang and Ploetz, Thomas and Xiao, Yu},
-  journal = {arXiv preprint arXiv:ARXIV_ID_TBD},
+  journal = {arXiv preprint arXiv:2604.21668},
   year    = {2026}
 }