Update 1_3_Real_Case_use_case.rst

Fixed ref: http://bazaar.launchpad.net/~openerp-community/openobject-doc/6.1/revision/1869/i18n/zh_hans/source/book/1/1_3_Real_Case/1_3_Real_Case_use_case.rst#i18n/zh_hans/source/book/1/1_3_Real_Case/1_3_Real_Case_use_case.rst