Adjust instructions for UTXO dataset creation

--nocleanup is sufficient to leave the data directories behind. --noshutdown is
only useful if you want to inspect the nodes afterwards, and you'd need to
manually shut down both nodes before copying the data directories.

qa/zcash/create_wallet_200k_utxos.py

@@ -9,7 +9,7 @@
 # To use:
 # - Copy to qa/rpc-tests/wallet_large.py
 # - Add wallet_large.py to RPC tests list
-# - ./qa/pull-tester/rpc-tests.sh wallet_large --noshutdown
+# - ./qa/pull-tester/rpc-tests.sh wallet_large --nocleanup
 # - Archive the resulting /tmp/test###### directory
 #