faq: stop mentioning bitcoin's impl detail

"Using Coinbase" is an implementation detail that could lead
people to think they need a Coinbase account for getting the
payout, which is not true because a bitcoin address is the
only requirement (Coinbase is just the USD->BTC conversion
service), as pointed out in gratipay#310.

www/about/faq.html.spt

@@ -83,7 +83,7 @@ title = "Frequently Asked Questions"
 
         <dt>Can I use bitcoin?</dt>
 
-        <dd>Sort of. We currently support manual payouts using Coinbase. 
+        <dd>Sort of. We currently support manual payouts.
         However, you cannot use bitcoin to pay in. The challenges with bitcoin 
         are handling recurring payments,
         and transparently converting to and from fiat currency. You're welcome