chore: fix const_tag_invalid_placement error msg

was missing from #14993

Author: dummdidumm

packages/svelte/messages/compile-errors/template.md

@@ -118,7 +118,7 @@
 
 ## const_tag_invalid_placement
 
-> `{@const}` must be the immediate child of `{#snippet}`, `{#if}`, `{:else if}`, `{:else}`, `{#each}`, `{:then}`, `{:catch}`, `<svelte:fragment>` or `<Component>`
+> `{@const}` must be the immediate child of `{#snippet}`, `{#if}`, `{:else if}`, `{:else}`, `{#each}`, `{:then}`, `{:catch}`, `<svelte:fragment>`, `<svelte:boundary>` or `<Component>`
 
 ## debug_tag_invalid_arguments