Fix typoed cuttlefish name

In the design section I noticed that the name of the app that helps you find icons, Cuttlefish was mistyped as Cuttelfish. This also affects the link on the button that allows to install the application using Discover.

In this simple merge request i fixed this issue on all places i could find (luckily it was only affecting the content/design.md file).

Do let me know if I'm not meeting the contribution guidelines, I tried looking for them but couldn't find a specific document detailing requirements for contributors so I tried to follow what was done in the previous merge requests that have been opened and merged on this project.