fix: Failing doctest example for new mempool

The mempool must now be initialized with a tip digest in order to keep
track of blocks for which the mempools transactions are valid. This is
introduced to allow the mempool to handle reorganizations without
containing invalid transactions.

src/models/state/mempool.rs

@@ -415,7 +415,9 @@ impl Mempool {
     /// use neptune_core::models::state::mempool::Mempool;
     /// use bytesize::ByteSize;
     ///
-    /// let mempool = Mempool::new(ByteSize::gb(1));
+    /// let network = Network::Main;
+    /// let genesis_block = Block::genesis_block(network);
+    /// let mempool = Mempool::new(ByteSize::gb(1), genesis_block);
     /// // insert transactions here.
     /// let mut most_valuable_transactions = vec![];
     /// for (transaction_digest, fee_density) in mempool.get_sorted_iter() {