fix: fix the problem of the copy button. (#220)

Author: jaywcjlove

core/src/styles/markdown.less

@@ -1002,9 +1002,7 @@ body[data-color-mode*='light'] {
 
 .wmde-markdown {
   pre {
-    &[class*='language-'] {
-      position: relative;
-    }
+    position: relative;
     .copied {
       visibility: hidden;
       display: flex;