Add missing SIGHASH flag to BIP 143 Example

[davidgumberg]

The first example (Native P2WPKH) signature in BIP 143 was missing a SIGHASH flag. The signature scheme requires a DER encoded signature with a one-byte SIGHASH/'hash type' flag appended. (Excepting schnorr signatures which can omit a flag to implicitly signal SIGHASH_DEFAULT)

This pull request corrects the example to include the 0x01 / SIGHASH_ALL flag.

[lorenzolfm]

ACK, since all other examples have the sighash flag appended in the signature. Either this or include a new field "sighash flag" and exclude the sighash flag from all other signature examples

[murchandamus]

Duplicate of #748