docs: Update for mempool v3 module.

docs/README.md

@@ -198,7 +198,7 @@ The following versioned modules are provided by dcrd repository:
     Implements the edwards25519 twisted Edwards curve
 * [database/v2](https://github.com/decred/dcrd/tree/master/database) -
   Provides a database interface for the Decred block chain
-* [mempool/v2](https://github.com/decred/dcrd/tree/master/mempool) - Provides a
+* [mempool/v3](https://github.com/decred/dcrd/tree/master/mempool) - Provides a
   policy-enforced pool of unmined Decred transactions
 * [dcrutil/v2](https://github.com/decred/dcrd/tree/master/dcrutil) - Provides
   Decred-specific convenience functions and types

docs/assets/module_hierarchy.gv

@@ -21,7 +21,7 @@ digraph {
 	standalone [label="blockchain/standalone" fillcolor=firebrick4]
 	gcs [label="gcs" fillcolor=gold]
 	blockchain [label="blockchain/v2" fillcolor=orchid]
-	mempool [label="mempool/v2" fillcolor=slategray]
+	mempool [label="mempool/v3" fillcolor=slategray]
 	mining [label="mining/v2" fillcolor=chartreuse]
 	lru  [label="lru" fillcolor=royalblue3]
 	peer [label="peer/v2" fillcolor=khaki]

docs/assets/module_hierarchy.svg

@@ -263,7 +263,7 @@
 <!-- mempool -->
 <g id="node20" class="node"><title>mempool</title>
 <path fill="slategray" stroke="black" d="M448.5,-108C448.5,-108 386.5,-108 386.5,-108 380.5,-108 374.5,-102 374.5,-96 374.5,-96 374.5,-84 374.5,-84 374.5,-78 380.5,-72 386.5,-72 386.5,-72 448.5,-72 448.5,-72 454.5,-72 460.5,-78 460.5,-84 460.5,-84 460.5,-96 460.5,-96 460.5,-102 454.5,-108 448.5,-108"/>
-<text text-anchor="middle" x="417.5" y="-86.3" font-family="Times New Roman,serif" font-size="14.00">mempool/v2</text>
+<text text-anchor="middle" x="417.5" y="-86.3" font-family="Times New Roman,serif" font-size="14.00">mempool/v3</text>
 </g>
 <!-- mempool&#45;&gt;dcrd -->
 <g id="edge27" class="edge"><title>mempool&#45;&gt;dcrd</title>