Update zh.ts

Update:

<message>
          <location filename="../billboard-ui.qml" line="199"/>
          <source>Brackets: {}</source>
 -        <translation type="unfinished"></translation>
 +        <translation type="unfinished">括号:{}</translation>
      </message>
      <message>
          <location filename="../billboard-ui.qml" line="200"/>
          <source>Brackets: {{}}</source>
 -        <translation type="unfinished"></translation>
 +        <translation type="unfinished">括号:{{}}</translation>
      </message>
      <message>
          <location filename="../billboard-ui.qml" line="201"/>
          <source>Embed image: &lt;&lt;&gt;&gt;</source>
 -        <translation type="unfinished"></translation>
 +        <translation type="unfinished">插入图片</translation>
      </message>