Tutorial image paths src/assets vs app/assets

[dap6000]

All the tutorial pages use the src/assets path for the image files. However, the repo for the tutorial app source code uses app/assets for three options:

1. Plain
2. Svelte
3. Vue

The tutorial text should be brought in line with the actual project structure in these three files:

1. tutorial/plain.md
2. tutorial/svelte.md
3. tutorial/vue.md

The existing src/assets text is correct for both Angular and React.