[docs] fix sentence of <svelte:element>

[baseballyama]

If the tag is changed, the children will be preserved unless there's a transition attached to the element.

Child elements are always regenerated when tag is changed. So I removed this sentence.

If this has a nullish value, a warning will be logged in development mode.

Now the warning is not showing. nullish value is officially supported.
So I updated the sentence.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with [feat], [fix], [chore], or [docs].
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[ignatiusmb]

LGTM 👍