Update prospectus for 2020

[itzsaga]

The Prospectus page needs to be updated for 2020 😃.

[lapinell]

@itzsaga I'm getting an "error not found 404" when I click on that link and it's in plain text... not even a normal 404..

[kwelch]

Here is the correct link: https://github.com/undergroundjs/website/blob/master/src/pages/become-a-sponsor.js

I also updated the one in the summary