trivial: fix wording and links in bolt03 (#1192)

* bolt03: fix link to `bitcoind`'s `policy.cpp` file

The original link is broken, we now replace it with a permalink.

* bolt03: remove 0 `txout` count in closing tx

Txns cannot have zero outputs.

* bolt03: remove wrong ref to `remotepubkey` and fix format

`remotepubkey` has been placed at the wrong place, also fixes the format
so it's easier to read.

03-transactions.md

@@ -360,7 +360,7 @@ Note that there are two possible variants for each node.
    * `txin[0]` sequence: 0xFFFFFFFF
    * `txin[0]` script bytes: 0
    * `txin[0]` witness: `0 <signature_for_pubkey1> <signature_for_pubkey2>`
-* txout count: 0, 1 or 2
+* txout count: 1 or 2
    * `txout` amount: final balance to be paid to one node (minus `fee_satoshis` from `closing_signed`, if this peer funded the channel)
    * `txout` script: as specified in that node's `scriptpubkey` in its `shutdown` message
 
@@ -514,7 +514,7 @@ Bitcoin Core defines the following dust thresholds:
 - pay to witness script hash (p2wsh): 330 satoshis
 - unknown segwit versions: 354 satoshis
 
-The rationale of this calculation (implemented [here](https://github.com/bitcoin/bitcoin/blob/0.21/src/policy/policy.cpp))
+The rationale of this calculation (implemented [here](https://github.com/bitcoin/bitcoin/blob/2aff9a36c352640a263e8b5de469710f7e80eb54/src/policy/policy.cpp#L28))
 is explained in the following sections.
 
 In all these sections, the calculations are done with a feerate of 3000 sat/kB
@@ -673,12 +673,11 @@ These pubkeys are simply generated by addition from their base points:
 
 	pubkey = basepoint + SHA256(per_commitment_point || basepoint) * G
 
-The `localpubkey` uses the local node's `payment_basepoint`;
-The `remotepubkey` uses the remote node's `payment_basepoint`;
-the `local_htlcpubkey` uses the local node's `htlc_basepoint`;
-the `remote_htlcpubkey` uses the remote node's `htlc_basepoint`;
-the `local_delayedpubkey` uses the local node's `delayed_payment_basepoint`;
-and the `remote_delayedpubkey` uses the remote node's `delayed_payment_basepoint`.
+- The `localpubkey` uses the local node's `payment_basepoint`;
+- The `local_htlcpubkey` uses the local node's `htlc_basepoint`;
+- The `remote_htlcpubkey` uses the remote node's `htlc_basepoint`;
+- The `local_delayedpubkey` uses the local node's `delayed_payment_basepoint`;
+- The `remote_delayedpubkey` uses the remote node's `delayed_payment_basepoint`.
 
 The corresponding private keys can be similarly derived, if the basepoint
 secrets are known (i.e. the private keys corresponding to `localpubkey`, `local_htlcpubkey`, and `local_delayedpubkey` only):