Update sphere.py

Fixed tabs to hopefully resolve the flake8 error

src/geometry/sphere.py

@@ -2,22 +2,22 @@
 
 def calculate_volume(r):
   """ 
-    Calculate the volume of a sphere. 
+  Calculate the volume of a sphere. 
 
-    This uses the standard mathematical calculation: 
+  This uses the standard mathematical calculation: 
 
-    $\(4/3) pi r ^3$
+  $\(4/3) pi r ^3$
 
-    Parameters
-    ----------
-    r : float, the radius 
+  Parameters
+  ----------
+  r : float, the radius 
 
-    Returns
-    -------
-    float : the volume of the sphere
-    """
-    volume = (4/3) * pi * r **3
-    return volume
+  Returns
+  -------
+  float : the volume of the sphere
+  """
+  volume = (4/3) * pi * r **3
+  return volume
 
 
 def calculate_surface_area(r):