BLS proof-of-possession command needs updating in validator quickstart

[asaj]

Expected Behavior

account proof-of-possession takes two arguments now. The first is the address from which the BLS key is derived. The second is the address that the BLS key signs as a proof-of-possession.

When not authorizing separate validator signers, these arguments are both $CELO_VALIDATOR_ADDRESS. The documentation should be updated to reflect this change.

Current Behavior

Currently takes one argument