Change the Discord message to target contributors

[kenzieschmoll]

The link to give people if they want to join the server is https://github.com/flutter/flutter/wiki/Chat. in general the Discord server is expected to be primarily for contributors, not users (we don't currently have enough moderation bandwidth to handle users also using our discord).

In this dialog, we should say, "Want to contribute? Please see our CONTRIBUTING.md guide or connect with us on Discord."

for the "Discord" link we should point to https://github.com/flutter/flutter/wiki/Chat