Update Survival analysis with lifelines.rst
Added  'exponential Greenwood CI' to the documents. The link was also dead. I included another link about the method instead.
@@ -259,7 +259,7 @@ see that very few leaders make it past 20 years in office. Of course,
like all good stats, we need to report how uncertain we are about these
point estimates, i.e., we need confidence intervals. They are computed in
the call to ``fit``, and located under the ``confidence_interval_``
-property. (The mathematics are found in `these notes <http://courses.nus.edu.sg/course/stacar/internet/st3242/handouts/notes2.pdf>`_.)
+property. (The method uses exponential Greenwood confidence interval. The mathematics are found in `these notes <https://www.math.wustl.edu/%7Esawyer/handouts/greenwood.pdf>`_.)
.. math:: S(t) = Pr( T > t)
