Replace signing with signature before validation

[jonasnick]

Actually we renamed validation to verification in bip-schnorr, so we may want to do this here was well for consistency.

Not necessarily because taproot signature validation does much more than just signature verification.

[real-or-random]

Also here in the first sentence of this paragraph I'm not sure:

https://github.com/sipa/bips/blob/bip-schnorr/bip-tapscript.mediawiki#transaction-digest

To be honest, I can't even parse the first clause of that sentence.

[OrfeasLitos]

Also here in the first sentence of this paragraph I'm not sure:

https://github.com/sipa/bips/blob/bip-schnorr/bip-tapscript.mediawiki#transaction-digest

To be honest, I can't even parse the first clause of that sentence.

Could be rephrased to "As the message used in the signature verification of signature opcodes"