Fix URL in CONTRIBUTING.md (material-extensions#2505)

Related to material-extensions#2504

Update the URL for the color palette in `CONTRIBUTING.md` to point to the correct URL.

* Change the URL on line 122 from `https://pkief.com/material-color-converter/` to `https://pkief.github.io/material-color-converter/`.

CONTRIBUTING.md

@@ -119,7 +119,7 @@ When designing folder icons there are also a few points to consider. A folder ic
 
 <img src="./images/how-tos/folder-icon-parts.svg" alt="An example of a folder" width="300px" />
 
-For the motive, only colors from the second row in the [color palette](https://pkief.com/material-color-converter/) are allowed. For the background choose a slightly darker hue (mostly in rows 4-6 in the palette).
+For the motive, only colors from the second row in the [color palette](https://pkief.github.io/material-color-converter/) are allowed. For the background choose a slightly darker hue (mostly in rows 4-6 in the palette).
 
 <img src="./images/how-tos/pick-folder-colors.png" alt="Material color palette" width="500px" />