Update send_onchain.md

src/guide/send_onchain.md

@@ -142,6 +142,7 @@ Fetching the fees also tells you what is the range of amounts the service allows
 </section>
 </custom-tabs>
 
+## Send all funds 
 In case you want to drain your channels you need to know the maximum sendable amount to an on-chain address:
 
 <custom-tabs category="lang">
@@ -354,4 +355,4 @@ If the reverse swap is successful, you'll get the on-chain payment on your desti
 change from pending to settled.
 
 If however something goes wrong at any point in the process, the initial HODL payment will be cancelled and the funds in
-your Breez SDK wallet will be unlocked.
+your Breez SDK wallet will be unlocked.