Its an interesting question!
Fixing small things like the above sentence would probably be fine. Depending on the context of your community.
Completely proofreading/fixing someone’s post might not be perceived as very friendly. But low-quality, badly written posts will also harm your community in the long-term.
It’s a tough balance, but doing it with the understanding/support of your existing community members does make sense. If people want to belong to your community… they have to abide by the standards of your community.
On the other hand, if this is your personal opinion, which isn’t shared by your members… then no, probably better to let it slide.