bip-373: Fix GLOBAL_XPUB key name and clean up compressed vs x-only note #2007
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Replaced incorrect PSBT key name PSBT_IN_GLOBAL_XPUB with the canonical PSBT_GLOBAL_XPUB (per BIP 174). This corrects scope naming (GLOBAL vs per-input) and aligns with existing PSBT specifications.
Removed duplicated/garbled sentence and punctuation in the reference note explaining why compressed pubkeys are used instead of x-only. The note now clearly states the evenness-byte requirement and fingerprint rationale without repetition.