Update "valid" to "normal" (#356)

...since the shorthand characters `:` and `@` are technically valid in HTML.

Author: chrisvfritz

src/guide/syntax.md

@@ -169,4 +169,4 @@ or
 <a @click="doSomething"></a>
 ```
 
-They may look a bit different from "valid" HTML, but all Vue.js supported browsers can parse it correctly, and they do not appear in the final rendered markup. The shorthand syntax is totally optional, but you will likely appreciate it when you learn more about its usage later.
+They may look a bit different from normal HTML, but all Vue.js supported browsers can parse it correctly, and they do not appear in the final rendered markup. The shorthand syntax is totally optional, but you will likely appreciate it when you learn more about its usage later.