Use desromech's abstract-gpu repo & minor change to readme

[anthonyc-r]

I've updated the readme, as it seems loading this repo in fails on pharo 9, but is successful with pharo 10 - so might confuse some. In addition I've updated the abstract-gpu dependency to point to desromech's repo, as this still contains binary builds. Without this, although Woden loads in, I've found that attempting to run an example fails.

Not sure why github is saying Sysmel changed, as looking at this file on this repo's master branch, it hasn't!