Update CHANGELOG.md

Author: baseballyama

CHANGELOG.md

@@ -18,6 +18,7 @@
 * Improve performance of custom element data setting in `<svelte:element>` ([#7869](https://github.com/sveltejs/svelte/pull/7869))
 * Supports nullish values for component event handlers ([#7568](https://github.com/sveltejs/svelte/issues/7568))
 * Better error message when specifying an invalid value for `<svelte:component this={...}>` ([#7550](https://github.com/sveltejs/svelte/issues/7550))
+* Fix to call component unmount if a component is mounted and then immediately unmounted ([#7817](https://github.com/sveltejs/svelte/issues/7817))
 
 ## 3.50.1