Fix a reference (networkx#6977)

* fix ref

* add url

Author: peijenburg

networkx/generators/classic.py

@@ -433,8 +433,9 @@ def dorogovtsev_goltsev_mendes_graph(n, create_using=None):
 
     References
     ----------
-    .. [1] Dorogotsev S.N., Goltsev A.V., and Mendes J.F.F "Pseudofractal
-       Scale-free Web". arXiv:cond-mat/0112143
+    .. [1] S. N. Dorogovtsev, A. V. Goltsev and J. F. F. Mendes,
+        "Pseudofractal scale-free web", Physical Review E 65, 066122, 2002.
+        https://arxiv.org/pdf/cond-mat/0112143.pdf
     """
     G = empty_graph(0, create_using)
     if G.is_directed():