wireguard: global: fix spelling mistakes in comments

This fixes two spelling errors in source code comments.

Signed-off-by: Josh Soref <jsoref@gmail.com>
[Jason: rewrote commit message]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>