Fix typo

Author: BirkhoffLee

index.html

@@ -23,7 +23,7 @@
   <div id="back_to_top">back to top</div>
   <div id="edit">edit</div>
   <div id="loading">Loading ...</div>
-  <div id="error">Opps! ... File not found!</div>
+  <div id="error">Oops! ... File not found!</div>
   <div id="flip"><div id="pageup">上一章</div><div id="pagedown">下一章</div></div>
   <div class="progress-indicator-2"></div>