[bug-fix] Fix typo (#5035)

* Fix typo

* Add test

ml-agents/mlagents/trainers/buffer.py

             dimension is equal to the length of the AgentBufferField.
         """
         if len(self) > 0 and not isinstance(self[0], list):
-            return np.asanyarray(self, dytpe=dtype)
+            return np.asanyarray(self, dtype=dtype)
 
         shape = None
         for _entry in self:

ml-agents/mlagents/trainers/tests/test_buffer.py

     assert np.array_equal(padded[0], np.array([1, 1, 1, 1]))
     assert np.array_equal(padded[1], np.array([2, 2, 3, 3]))
 
+    # Make sure it doesn't fail when the field isn't a list
+    padded_a = a.padded_to_batch()
+    assert np.array_equal(padded_a, a)
+
 
 def fakerandint(values):
     return 19