fix reader page to use the background script as a clipboard source instead of breaking the selection all the time

reader.html

@@ -14,7 +14,7 @@ html
     overflow-y: scroll;
 }
 </style>
-<script src="reader.js"></script>
+<script src="reader.js" charset="UTF-8"></script>
 </head>
 
 <body lang="ja">