docs/descriptors.md: Remove hardened marker in the path after xpub

Summary:
As described in "Key origin identification" section, a descriptor
that has hardened derivation after xpub does not let you compute scripts
without access to the corresponding private keys. Such a descriptor is
practically useless.

The text after the descriptor said "with child key *1'/2* of the
specified xpub", and clearly an xpub cannot have "child key" with
hardened derivation. Therefore it makes sense to fix this inconsistency
to not confuse the reader of the doc

This is a backport of core#20329

Test Plan: proofreading

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10806