style: update markdown style.

Author: jaywcjlove

src/styles/markdown.less

@@ -70,15 +70,13 @@
   }
 
   pre code {
-    background: none;
-    overflow-wrap: normal;
-    white-space: inherit;
-    tab-size: 2;
-    padding: 16px;
-    font-size: 95%;
-    line-height: 1.5;
     display: block;
-    text-shadow: 0 1px #fff;
+    padding: 16px;
+    overflow: auto;
+    font-size: 85%;
+    line-height: 1.45;
+    background-color: #f6f8fa;
+    border-radius: 6px;
   }
 
   pre {