fix a typo.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@34763 20353a03-c40f-0410-a6d1-a30d3c3de9de

bt5/erp5_trade_proxy_field_legacy/bt/description

-This business template holds obsolete fields libraries. It is only usefull for custom projects that still use old libraries.
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+This business template holds obsolete fields libraries. It is only useful for custom projects that still use old libraries.
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bt5/erp5_trade_proxy_field_legacy/bt/revision

-7
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+8
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