Fixed old copy/paste foldl typo in docs.

[oubiwann]

Side note: I was up until 2am last night reading through all the docs and came across this in the process. The reason for this was not diligence or any form of compulsion, rather being generally stunned by the beauty of Hackett. The last time I looked at the project, it was still Rascal (!) ... I can't remember the state of things from that time, but where it is now blows me away; couldn't stop reading code (ended up falling asleep with the laptop on my chest).