fix HTML errors in 2018/yang/spoiler.html and 2019/yang/spoiler.html

2018/yang/spoiler.html

@@ -1,8 +1,8 @@
 <!DOCTYPE html>
 <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
 <head>
-<meta>
-<style type="text/css">
+<meta charset="utf-8">
+<style>
 .tmp{color:#00f}.final{color:#000}</style>
 </head><body>
 <noscript><span style="color:red">JavaScript required</span></noscript>