BOLT 2: link to BOLT 7 when referring announcement_signature msg

This commit modifies the “Normal Operation” summarization by including
a link to BOLT #7 when mentioning the `announcement_signature` message.
Previously a reader would need to search other documents to figure out
what an `announcement_signature` was, and its purpose.

Author: Roasbeef

02-peer-protocol.md

@@ -426,7 +426,7 @@ reason to pay a premium for rapid processing.
 
 ## Normal Operation
 
-Once both nodes have exchanged `funding_locked` (and optionally `announcement_signatures`), the channel can be used to make payments via Hash TimeLocked Contracts.
+Once both nodes have exchanged `funding_locked` (and optionally [`announcement_signatures`](https://github.com/lightningnetwork/lightning-rfc/blob/master/07-routing-gossip.md#the-announcement_signatures-message)), the channel can be used to make payments via Hash TimeLocked Contracts.
 
 Changes are sent in batches: one or more `update_` messages are sent before a
 `commitment_signed` message, as in the following diagram: