tweak spelling mistake

src/directive/clipboard/clipboard.js

@@ -1,7 +1,7 @@
 // Inspired by https://github.com/Inndy/vue-clipboard2
 const Clipboard = require('clipboard')
 if (!Clipboard) {
-  throw new Error('you shold npm install `clipboard` --save at first ')
+  throw new Error('you should npm install `clipboard` --save at first ')
 }
 
 export default {