docs: Update for blockchain v4 module.

This updates the docs/README.md file, module hierarchy graphviz, and
module hierarchy diagram to reflect the new module version.

docs/README.md

@@ -182,7 +182,7 @@ The following versioned modules are provided by dcrd repository:
   Decred wire protocol
 * [peer/v3](https://github.com/decred/dcrd/tree/master/peer) - Provides a common
   base for creating and managing Decred network peers
-* [blockchain/v3](https://github.com/decred/dcrd/tree/master/blockchain) -
+* [blockchain/v4](https://github.com/decred/dcrd/tree/master/blockchain) -
   Implements Decred block handling and chain selection rules
   * [stake/v4](https://github.com/decred/dcrd/tree/master/blockchain/stake) -
     Provides an API for working with stake transactions and other portions

docs/assets/module_hierarchy.gv

@@ -23,7 +23,7 @@ digraph {
 	stake [label="blockchain/stake/v4" fillcolor=bisque]
 	standalone [label="blockchain/standalone/v2" fillcolor=firebrick4]
 	gcs [label="gcs/v3" fillcolor=gold]
-	blockchain [label="blockchain/v3" fillcolor=orchid]
+	blockchain [label="blockchain/v4" fillcolor=orchid]
 	lru [label="lru" fillcolor=royalblue3]
 	peer [label="peer/v3" fillcolor=khaki]
 	rpcclient [label="rpcclient/v6" fillcolor=mediumseagreen]