Fix broken link to Trac

Signed-off-by: Benoit Donneaux <benoit@leastauthority.com>

src/index.html

@@ -74,7 +74,7 @@ about_list:
     btn_class: "btn--primary"
   - title: "Donation"
     excerpt: 'You can donate to the Tahoe-LAFS project via BitCoin and PayPal.<br/><br/>
-    <a href="/trac/tahoe-lafs/wiki/BitCoinPage" target="_blank">
+    <a href="https://tahoe-lafs.org/trac/tahoe-lafs/wiki/BitCoinPage" target="_blank">
     <img src="assets/images/bitcoin.png"></a>
     <a href="https://www.paypal.com/donate?cmd=_s-xclick&hosted_button_id=XQM2LB4UC8V2J" target="_blank">
     <img src="assets/images/paypal.png"></a>'