PathPlanning/InformedRRTStar/informed_rrt_star.py: Fix hard coded graph axis

[wltjr]

Replacing the hard coded graph axis values with class variables

• Use self.min_rand, self.max_rand variable values in place of -2, 15

Reference issue

What does this implement/fix?

This replaces hard coded axis values of -2, 15 with self.min_rand, self.max_rand class variables when drawing the graph

Additional information

This makes the function re-usable for varying sized graphs, beyond the example case in the main function

CheckList

• [ ] Did you add an unittest for your new example or defect fix?
• [ ] Did you add documents for your new example?
• [ ] All CIs are green? (You can check it after submitting)