Change defaultMessage for spoiler placeholder (#4329)

app/javascript/mastodon/features/compose/components/compose_form.js

@@ -20,7 +20,7 @@ import { length } from 'stringz';
 
 const messages = defineMessages({
   placeholder: { id: 'compose_form.placeholder', defaultMessage: 'What is on your mind?' },
-  spoiler_placeholder: { id: 'compose_form.spoiler_placeholder', defaultMessage: 'Content warning' },
+  spoiler_placeholder: { id: 'compose_form.spoiler_placeholder', defaultMessage: 'Write your warning here' },
   publish: { id: 'compose_form.publish', defaultMessage: 'Toot' },
   publishLoud: { id: 'compose_form.publish_loud', defaultMessage: '{publish}!' },
 });