Update docstring to include 'a' parameter

Added parameter description for 'a' in the docstring.

Author: caph1993

ot/stochastic.py

@@ -178,6 +178,8 @@ def averaged_sgd_entropic_transport(
 
     Parameters
     ----------
+    a : ndarray, shape (ns,)
+        source measure
     b : ndarray, shape (nt,)
         target measure
     M : ndarray, shape (ns, nt)