Update snow consensus doc post X-chain linearization (#1703)

Author: exdx

snow/README.md

@@ -16,7 +16,9 @@ graph LR
 
 ## Intro
 
-The Avalanche network consists of 3 built-in blockchains: X-Chain, C-Chain, and P-Chain. The X-Chain is used to manage assets and uses the Avalanche consensus protocol. The C-Chain is used to create and interact with smart contracts and uses the Snowman consensus protocol. The P-Chain is used to coordinate validators and stake and also uses the Snowman consensus protocol. At the time of writing, the Avalanche network has ~1200 validators. A set of validators makes up a subnet. Subnets can validate 1 or more chains. It is a common misconception that 1 subnet = 1 chain and this is shown by the primary subnet of Avalanche which is made up of the X-Chain, C-Chain, and P-Chain.
+The Avalanche primary network consists of 3 built-in blockchains: the X-Chain, C-Chain, and P-Chain. All three chains rely on the Snowman consensus protocol. The X-Chain, which previously used DAG-based Avalanche consensus, was upgraded to Snowman in the Cortina network update. 
+
+The X-Chain is used to manage assets. The C-Chain is used to create and interact with smart contracts. The P-Chain is used to coordinate validators and stake. At the time of writing, the Avalanche network has ~1200 validators. A set of validators makes up a subnet. Subnets can validate 1 or more chains. It is a common misconception that 1 subnet = 1 chain and this is shown by the primary subnet of Avalanche which is made up of the X-Chain, C-Chain, and P-Chain.
 
 A node in the Avalanche network can either be a validator or a non-validator. A validator stakes AVAX tokens and participates in consensus to earn rewards. A non-validator does not participate in consensus or have any AVAX staked but is used as a public API. Both validators and non-validator need to have their own copy of the chain and to know the current state of the mempool. At the time of writing, there are ~1200 validators and ~1800 non-validator.