Sketch for Material-UI link fix in Documenation

[bramick]

https://material-ui.com/discover-more/related-projects/
Design Resources
• Sketch: "Sketch for Material-UI"

• The issue is present in the latest release.
• [x ] I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

"Sketch for Material-UI"
Current link: https://material-ui.com/store/item/sketch-react/

Expected Behavior 🤔

Correct link: https://material-ui.com/store/items/sketch-react/

Steps to Reproduce 🕹

Steps:

1. Go to page https://material-ui.com/discover-more/related-projects/
2. Select "Sketch for Material-UI". Goes to page not found

Tech	Version
Material-UI	v4.9.5

[oliviertassinari]

Thanks, confirmed, this was our last 404 :)