src/doc/en/reference/references/index.rst: Add Sey1980

Author: Matthias Koeppe

src/doc/en/reference/references/index.rst

@@ -5545,7 +5545,10 @@ REFERENCES:
              spaces* in Scientific contributions in honor of Mirian
              Andrés Gómez, pp 507-519, Univ. La Rioja Serv. Publ., Logroño (2010).
 
-.. [Sey1981] \P. D. Seymour, Nowhere-zero 6-flows, J. Comb. Theory Ser B,
+.. [Sey1980] \P. D. Seymour, *Decomposition of regular matroids*,
+             J. Comb. Theory Ser B, 28 (1980), 305-359.
+
+.. [Sey1981] \P. D. Seymour, *Nowhere-zero 6-flows*, J. Comb. Theory Ser B,
              30 (1981), 130-135. :doi:`10.1016/0095-8956(81)90058-7`
 
 .. [SH1995] \C. P. Schnorr and H. H. Hörner. *Attacking the