Example path for images needs updating

[AnthonyAndroulakis]

I noticed the examples have the niivue image path listed as https://github.com/niivue/niivue/raw/main/demos/images/, which results in this not found error:

changing the path to path = "https://github.com/niivue/niivue/raw/main/packages/niivue/demos/images/" fixes this.

[christian-oreilly]

Hey @AnthonyAndroulakis! Thanks for pointing this out! PR #80 which should merge soon will simplify the use of example datasets and should fix this issue at the same time. I'll flag this issue in that PR so that we make sure it does. We should also ensure the CI runs the example notebooks when commits are done on a PR so that any future changes that break notebooks get automatically flagged and fixed before merging!