Fix typo in code snippet for extend

the SCSS snippet sets the "border" property of .message,
but the resulting CSS snippet has "border-color" (should be "border")
(and obviously you can't use "1px solid" on border-color)

source/code-snippets/_homepage-extend-css.md

@@ -1,6 +1,6 @@
 ```css
 .message, .success, .error, .warning {
-  border-color: 1px solid #cccccc;
+  border: 1px solid #cccccc;
   padding: 10px;
   color: #333;
 }