add journal to nnpdf4.0 code paper reference in docs

Author: RoyStegeman

doc/sphinx/source/references.bib

@@ -633,14 +633,19 @@ @article{nnpdf40
  primaryClass = {hep-ph}
 }
 
-@article{nnpdf40code,
+@article{NNPDF:2021uiq,
     author = "Ball, Richard D. and others",
+    collaboration = "NNPDF",
     title = "{An open-source machine learning framework for global analyses of parton distributions}",
     eprint = "2109.02671",
     archivePrefix = "arXiv",
     primaryClass = "hep-ph",
     reportNumber = "Edinburgh 2021/13, Nikhef-2021-020, TIF-UNIMI-2021-12",
-    month = "9",
+    doi = "10.1140/epjc/s10052-021-09747-9",
+    journal = "Eur. Phys. J. C",
+    volume = "81",
+    number = "10",
+    pages = "958",
     year = "2021"
 }