doc: update docs/css.md (#849)

* 删除一个错误字符

* Update css.md

Author: kele527

docs/css.md

@@ -156,7 +156,7 @@ div {
 ```css
 #container {
   display: grid;
-s  grid: repeat(2, 60px) / auto-flow 80px;
+  grid: repeat(2, 60px) / auto-flow 80px;
 }
 #container > div {
   background-color: #8ca0ff;