Update periods_and_fiscal_years.rst

Fixed ref: http://bazaar.launchpad.net/~openerp-community/openobject-doc/6.1/revision/1869/i18n/zh_hans/source/book/3/3_9/periods_and_fiscal_years.rst#i18n/zh_hans/source/book/3/3_9/periods_and_fiscal_years.rst