Update contribute.md

[ClaytonCurtis1947]

Corrected spelling of 'volunteers' as test of editing capability.

[macmarrum]

@ClaytonCurtis1947 This is great. Thank you for your first contribution.

BTW, as a rule, I try to use the imperative mood in commit messages, i.e. "correct" instead of "corrected" or "corrects". I highly recommend it.