List formatting in docs (#2939)

* list formatting in docs

* pep8

Author: max-sixty

xarray/core/groupby.py

@@ -519,6 +519,7 @@ def apply(self, func, shortcut=False, args=(), **kwargs):
 
         Apply uses heuristics (like `pandas.GroupBy.apply`) to figure out how
         to stack together the array. The rule is:
+
         1. If the dimension along which the group coordinate is defined is
            still in the first grouped array after applying `func`, then stack
            over this dimension.
@@ -661,6 +662,7 @@ def apply(self, func, args=(), **kwargs):
 
         Apply uses heuristics (like `pandas.GroupBy.apply`) to figure out how
         to stack together the datasets. The rule is:
+
         1. If the dimension along which the group coordinate is defined is
            still in the first grouped item after applying `func`, then stack
            over this dimension.