docs/website: fix Paypal account address

As noticed by Yann E. Morin, the address of the Paypal account is
@buildroot.org, not @lists.buildroot.org.

Reported-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>