doc: use kbd element in readline doc prose
The kbd element has already been added in the keybindings table, but we missed it in one line in the prose.
Checklist
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documentation is changed or added -
commit message follows commit guidelines
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
The kbd element has already been added in the keybindings table, but we missed it in one line in the prose.