Adding a hint in the tutorial pop-up in Rodan: choosing "rectangle" tool for selected region

Before submitting the corrected pixel.js job in Rodan, if the user needs to select the whole page in the "Select Region" layer, the user needs to choose the ["rectangle" tool](https://github.com/DDMAL/Pixel.js/wiki/Pixel.js-Tools#rectangle) rather than the ["select" tool](https://github.com/DDMAL/Pixel.js/wiki/Pixel.js-Tools#select) in Pixel; otherwise, nothing is selected since this the layer was empty. It would be nice to add this hint in the [pop-up tutorial in Pixel](https://github.com/DDMAL/Pixel.js/wiki/How-to-use-Pixel.js#tutorial-pop-up) in Rodan to avoid confusion.

Tutorial pop-up in Pixel in Rodan: