fix mail link on notedeck install webpage

[alltheseas]

https://damus.io/notedeck/install/ does not contain mailto:

The “contact us” button is not “mailto”. It tries to redirect you to “https://damus.io/notedeck/install/support@damus.io%E2%80%9D that doesn’t exist.

[linear]

SER-58 fix mail link on notedeck install webpage