docs/design-notes/IDL.md: fix typo: s/on only/only/
docs/design-notes/IDL.md: fix typo: s/on only/only/

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <20220118094416.242409-1-a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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