fix line_join to joint_type in example_scenes/basic.py (#3510)

Author: szchixy

example_scenes/basic.py

@@ -167,8 +167,8 @@ def construct(self):
 class LineJoints(Scene):
     def construct(self):
         t1 = Triangle()
-        t2 = Triangle(line_join=LineJointType.ROUND)
-        t3 = Triangle(line_join=LineJointType.BEVEL)
+        t2 = Triangle(joint_type=LineJointType.ROUND)
+        t3 = Triangle(joint_type=LineJointType.BEVEL)
 
         grp = VGroup(t1, t2, t3).arrange(RIGHT)
         grp.set(width=config.frame_width - 1)