style: fix font family of highlighted code (#1354)

src/sites/assets/styles/highlight.scss

@@ -13,6 +13,10 @@ code {
   -webkit-font-smoothing: auto;
   background-color: #fafafa;
   border-radius: 16px;
+
+  span {
+    font-family: $nutui-doc-code-font-family;
+  }
 }
 
 pre {