message: docs(Button): fix icon-position missing type
diff: @@ -111,7 +111,7 @@ Vue.use(Button);
  | color | Color, support linear-gradient | _string_ | - |
  | icon | Left Icon | _string_ | - |
  | icon-prefix `v2.6.0` | Icon className prefix | _string_ | `van-icon` |
  -| icon-position `v2.10.7` | Icon position, can be set to `right` | `left` |
  +| icon-position `v2.10.7` | Icon position, can be set to `right` | _string_ | `left` |
  | tag | HTML Tag | _string_ | `button` |
  | native-type | Native Type Attribute | _string_ | `''` |
  | plain | Whether to be plain button | _boolean_ | `false` |
  