fix cut and paste error in DHRobot.alpha method

Author: petercorke

roboticstoolbox/robot/DHRobot.py

@@ -461,7 +461,7 @@ def alpha(self):
         """
         v = []
         for i in range(self.n):
-            v.append(self.links[i].theta)
+            v.append(self.links[i].alpha)
         return v
 
     @property

tests/test_DHRobot.py

@@ -8,6 +8,7 @@
 import roboticstoolbox as rp
 import spatialmath as sm
 import unittest
+import math
 
 
 class TestDHRobot(unittest.TestCase):
@@ -1485,7 +1486,7 @@ def test_alpha(self):
         r0 = rp.models.DH.Puma560()
 
         nt.assert_array_almost_equal(
-            r0.alpha, [0., 0., 0., 0., 0., 0.])
+            r0.alpha, np.r_[1, 0, -1, 1, -1, 0] * math.pi / 2)
 
     def test_ets(self):
         panda = rp.models.DH.Panda()