docs: fix scss example typo

close #2272

Author: yyx990803

docs/guide/css.md

@@ -25,7 +25,7 @@ Then you can import the corresponding file types, or use them in `*.vue` files w
 
 ``` vue
 <style lang="scss">
-$color = red;
+$color: red;
 </style>
 ```
 

docs/ru/guide/css.md

@@ -25,7 +25,7 @@ npm install -D stylus-loader stylus
 
 ``` vue
 <style lang="scss">
-$color = red;
+$color: red;
 </style>
 ```
 

docs/zh/guide/css.md

@@ -25,7 +25,7 @@ npm install -D stylus-loader stylus
 
 ``` vue
 <style lang="scss">
-$color = red;
+$color: red;
 </style>
 ```