Update built-in-components.md (#780)

Author: Alex-Sokolov

src/api/built-in-components.md

@@ -39,7 +39,7 @@
   - `css` - `boolean`. Whether to apply CSS transition classes. Defaults to `true`. If set to `false`, will only trigger JavaScript hooks registered via component events.
   - `type` - `string`. Specifies the type of transition events to wait for to determine transition end timing. Available values are `"transition"` and `"animation"`. By default, it will automatically detect the type that has a longer duration.
   - `mode` - `string` Controls the timing sequence of leaving/entering transitions. Available modes are `"out-in"` and `"in-out"`; defaults to simultaneous.
-  - `duration` - `number | {`enter`: number,`leave`: number }`. Specifies the duration of transition. By default, Vue waits for the first `transitionend` or `animationend` event on the root transition element.
+  - `duration` - `number | { enter: number, leave: number }`. Specifies the duration of transition. By default, Vue waits for the first `transitionend` or `animationend` event on the root transition element.
   - `enter-from-class` - `string`
   - `leave-from-class` - `string`
   - `appear-class` - `string`