chore: update config example file

_config.yml.example

@@ -43,7 +43,7 @@ widgets:
 
 # Search
 search:
-    insight: true
+    insight: true # you need to install `hexo-generator-json-content` before using Insight Search
     swiftype: # enter swiftype install key here
     baidu: false # you need to disable other search engines to use Baidu search, options: true, false