Added server termination instruction log

Author: adamlui

utils/previewDocs.js

@@ -1,4 +1,6 @@
 #!/usr/bin/env node
 
 require('child_process').exec('docsify serve docs').stdout.on('data', data => {
-    console.log(data) ; require('open')('http://localhost:3000') })
+    console.log(data) ; console.log('To stop the server, press Ctrl+C')
+    require('open')('http://localhost:3000')
+})