redirect scheme
is indeed not available in HAProxy 1.4.24. Right now, it is available in HAProxy 1.5-dev13 and newer as well as in HAProxy 1.4.25 and newer, including the haproxy-1.4 master.
The documentation you had a look at was probably the one by Cyril Bonté which currently is generated from the 1.4 master, not the 1.4.24 release.
As such, you could either upgrade to one of the named versions or work around the limitation. A common workaround is to use redirect prefix
like this
redirect prefix https://domain.com if is_http { hdr(host) -i domain.com }
This rule has to be enumerated for each hostname where you want the redirect to happen.