Fix citation in pulse_lib.discrete (#3850)

qiskit/pulse/pulse_lib/discrete.py

@@ -446,9 +446,14 @@ def drag(duration: int, amp: complex, sigma: float, beta: float,
 
     If ``zero_ends == True``, the samples from :math:`g(x)` are remapped as in :meth:`gaussian`.
 
-    [1] Gambetta, J. M., Motzoi, F., Merkel, S. T. & Wilhelm, F. K.
-    Analytic control methods for high-fidelity unitary operations
-    in a weakly nonlinear oscillator. Phys. Rev. A 83, 012308 (2011).
+    References:
+        1. |citation1|_
+
+        .. _citation1: http://dx.doi.org/10.1103/PhysRevA.83.012308
+
+        .. |citation1| replace:: *Gambetta, J. M., Motzoi, F., Merkel, S. T. & Wilhelm, F. K.
+           "Analytic control methods for high-fidelity unitary operations
+           in a weakly nonlinear oscillator." Phys. Rev. A 83, 012308 (2011).*
 
     Args:
         duration: Duration of pulse. Must be greater than zero.