Update index.html

[cheeyuannn]

set minimum input length for message to avoid message that is too short and cannot provide enough information

[mckennapsean]

I can see the potential use-case, but why 40? Do people have issues with not enough being filled out? What if you just have a short legit message to send? Not sure. Might be worth seeing how browsers behave here too (it's best when doing validation on a form to communicate what a user needs to do to continue; not sure what html does here offhand, so I am legit curious...)