Guide: Closures: minor wording fixes
Guide: Closures fix suggested by @cakebaker
auto merge of #18340 : chastell/rust/guide_closures_fixes, r=steveklabnik

Some minor wording fixes to the Closures chapter; my brain tripped a few times when reading it, so I tried to come up with something a bit smoother. I’m not a native speaker, so please do review this critically.