DOC: specify link to frequencies (#14760)

(cherry picked from commit 2bd9c95)

Author: jorisvandenbossche

pandas/core/groupby.py

@@ -175,8 +175,8 @@ class Grouper(object):
     freq : string / frequency object, defaults to None
         This will groupby the specified frequency if the target selection
         (via key or level) is a datetime-like object. For full specification
-        of available frequencies, please see
-        `here <http://pandas.pydata.org/pandas-docs/stable/timeseries.html>`_.
+        of available frequencies, please see `here
+        <http://pandas.pydata.org/pandas-docs/stable/timeseries.html#offset-aliases>`_.
     axis : number/name of the axis, defaults to 0
     sort : boolean, default to False
         whether to sort the resulting labels