Fix `cumsum()` docstring
"For ``start_angle``" -> "For ``end_angle``"
@@ -66,7 +66,7 @@ def cumsum(field, include_zero=False):
will generate a ``CumSum`` expressions that sum the ``"angle"`` column
of a data source. For the ``start_angle`` value, the cumulative sums
- will start with a zero value. For ``start_angle``, no initial zero will
+ will start with a zero value. For ``end_angle``, no initial zero will
be added (i.e. the sums will start with the first angle value, and
include the last).
