fix: clarify markdown note placeholder

[chaochaoweb3]

Summary

• remove the misleading backtick from the note placeholder
• clarify that Markdown notes should start with the literal prefix

Fixes #678.

Test

• rg -n 'start with||use Markdown|use markdown' app/renderer/ui/edit-view/edit-view.vue
• git diff --check

Note: I did not run the full Electron/Vue typecheck because this fresh checkout has no node_modules installed and this patch only changes one UI string.