Update importing_blockchain.md

With the most recent gui release download for windows 10 from https://getmonero.org/downloads/ (Lithium Luna) the supported relevant command line option is:
 --guard-against-pwnage arg (=1)       Verify blocks and transactions during
                                        import (only disable if you exported
                                        the file yourself)

Since --verify 1 is not only not supported anymore, but is default behavior it should be removed from the guide.