Use correct source_path in example

[aaronayres35]

fixes #1432

This PR changes the search path the be the correct path to the directory that contains the image folder containing the images of interest.

[kitchoi]

May I also suggest adding a simple test to prevent this missing file link from happening again?
Something like this.

import os
import runpy
import unittest

#: Filename of the demo script
FILENAME = "Image_editor_demo.py"

#: Path of the demo script
DEMO_PATH = os.path.join(os.path.dirname(__file__), "..", FILENAME)


class TestImageEditorDemo(unittest.TestCase):

    def test_image_path_exists(self):
        search_path, = runpy.run_path(DEMO_PATH)["search_path"]
        self.assertTrue(os.path.exists(search_path))

[kitchoi]

If I run the script from the command line, even just search_path = [] would work, but that's using some default magic (with inspection on where the code is running from etc.). I believe this is the magic:
https://github.com/enthought/traits/blob/a0d0a70331ec202eb33b6439570175c869fe353c/traits/trait_base.py#L256-L305

That would not work when this example is run from within the demo application. join(dirname(__file__)) remove the assumption and dependency on the magic: It is more explicit, which I think is a good thing to do anyway. Now the comment makes it look worse than this actually is.

What do you think about rewording the comment to something along the line of # The images folder is in the same folder as this file?

[kitchoi]

Still LGTM. Thank you!