corrected reference

Author: Krishnadubey1008

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

@@ -5644,8 +5644,10 @@ REFERENCES:
               Congressus numerantium, 1994.
               Pages 97--110
 
-.. [Rea1968] Ronald C. Read, An improved method for computing the chromatic polynomials of
-             sparse graphs.
+.. [Rea1968] Ronald C. Read,
+             An improved method for computing the chromatic polynomials of sparse graphs,
+             Research Report CORR 87-20, C & O Dept. Univ. of Waterloo, 1987.
+             :doi:`10.1016/S0012-365X(97)90031-5`.
 
 .. [Rea2004] Nathan Reading.
              *Cambrian Lattices*.