blockchain: clarify MAX_TARGET by referencing bitcoin core source

change is no-op as the compact nBits form of both values are equal, that is:
```
>>> from electrum.blockchain import Blockchain
>>> MAX_TARGET1 = 0x00000000FFFF0000000000000000000000000000000000000000000000000000
>>> MAX_TARGET2 = 0x00000000ffffffffffffffffffffffffffffffffffffffffffffffffffffffff
>>> Blockchain.bits_to_target(Blockchain.target_to_bits(MAX_TARGET2)) == Blockchain.bits_to_target(Blockchain.target_to_bits(MAX_TARGET1))
True
```

electrum/blockchain.py

@@ -37,7 +37,9 @@ from .logging import get_logger, Logger
 _logger = get_logger(__name__)
 
 HEADER_SIZE = 80  # bytes
-MAX_TARGET = 0x00000000FFFF0000000000000000000000000000000000000000000000000000
+
+# see https://github.com/bitcoin/bitcoin/blob/feedb9c84e72e4fff489810a2bbeec09bcda5763/src/chainparams.cpp#L76
+MAX_TARGET = 0x00000000ffffffffffffffffffffffffffffffffffffffffffffffffffffffff  # compact: 0x1d00ffff
 
 
 class MissingHeader(Exception):