fixed placeholder action

src/const.js

@@ -34,7 +34,7 @@ export const actions = {
   setIndent: 'indent',
   setOutdent: 'outdent',
   setTitlePlaceholder: 'SET_TITLE_PLACEHOLDER',
-  setContentPlaceholder: 'SET_TITLE_PLACEHOLDER',
+  setContentPlaceholder: 'SET_CONTENT_PLACEHOLDER',
   setTitleFocusHandler: 'SET_TITLE_FOCUS_HANDLER',
   setContentFocusHandler: 'SET_CONTENT_FOCUS_HANDLER',
   prepareInsert: 'PREPARE_INSERT',