message: docs: update some examples of button and cell
diff: @@ -97,7 +97,7 @@ Vue.use(Cell).use(CellGroup);
  ### Vertical Center
  ```html
  -<van-cell center title="Cell title" value="Multiple lines content, multiple lines content" />
  +<van-cell center title="Cell title" value="Content" label="Description" />
  ```
  ## API
  