move reference to footnote

Author: guitorri

src/continuous_famous_systems.jl

@@ -1211,7 +1211,12 @@ where ``i,j=1,2 (i\\neq j)`` denote two oscillators.
 The two coupled Hindmarsh Rose element by chemical and electrical synapse.
 It is modelling the dynamics of a neuron's membrane potential.
 The default parameter values are taken from article "Dragon-king-like extreme events in
-coupled bursting neurons", DOI:https://doi.org/10.1103/PhysRevE.97.062311.
+coupled bursting neurons", from [^Mishra2018].
+
+[^Mirshra2018]:
+    A. Mishra, S. Saha, M. Vigneshwaran, P. Pal, T. Kapitaniak, and S. K. Dana,
+    "Dragon-king-like extreme events in coupled bursting neurons",
+    Phys. Rev. E, vol. 97, no. 6, p. 062311, Jun. 2018, doi: 10.1103/PhysRevE.97.062311.
 """
 function hindmarshrose_two_coupled(u0=[0.1, 0.2, 0.3, 0.4, 0.5, 0.6];
 			a = 1.0, b = 3.0, c=1.0, d = 5.0, r = 0.001, s = 4.0, xr = -1.6, I = 4.0,