更新preface.md 修复昨日merge导入的错误

昨天在旧的preface.md上修改完就直接pull request了,忘记先update一下,导致又出现了英文符":",实在抱歉!
Merge pull request #34 from diseng/patch-1

更新preface.md 修复昨日merge导入的错误