Fix an invalid link

[artemkorsakov]

There is an invalid link on the following page:
https://docs.scala-lang.org/sips/futures-promises.html

Fixed!

@SethTisue @adpi2 please, review this PR.

[SethTisue]

what's weird is that I had already fixed this myself, but it appears that some bot undid the fix: 7ec2ca6

before merging this, I'd like to understand what happened there so we can prevent it from recurring. @kyouko-taiga @hamzaremmal do you know anything about this "Scala Improvement Bot" ?

[hamzaremmal]

what's weird is that I had already fixed this myself, but it appears that some bot undid the fix: 7ec2ca6

It needs to be fixed in the SIP repository, not here. The SIP bot will synchronise the content of this repo with the content from the SIP repo. Therefore, any change you do here, will be overridden by the content in improvement-proposals

[kyouko-taiga]

I opened scala/improvement-proposals#106

[hamzaremmal]

I've just triggered the sync from the scala/improvement-proposals repo: 64d88df. Should be good now.

[SethTisue]

thanks all!