Fix drill_feature parameter mismatch

[obucklin]

Docstring and parameters don't match

`class DrillFeature(Feature):
"""Parametric drill hole to be made on an element.

Parameters
----------
plane : :class:`compas.geometry.Plane`
    The plane on which the drill hole is to be made.
diameter : float
    The diameter of the drill hole.
length : float
    The length (depth?) of the drill hole.

"""

def __init__(self, line, diameter, length, **kwargs):
    super(DrillFeature, self).__init__(**kwargs)
    self.line = line
    self.diameter = diameter
    self.length = length`

[chenkasirer]

DrillFeature will be removed once we're fully transitioned to the new BTLx workflow (very soon..)