Update JavaScript for...in syntax block (#5079)

Add missing angled brackets (`{ }`) missing from `for...in` syntax example.
Adjust HTML formatting in line with `for...of` syntax.

https://github.com/mdn/content/blob/f9d710480812705f184665a3bd42de55e9ab098f/files/en-us/web/javascript/reference/statements/for...of/index.html#L27-L30

Author: Jinksi

files/en-us/web/javascript/reference/statements/for...in/index.html

@@ -19,8 +19,10 @@
 
 <h2 id="Syntax">Syntax</h2>
 
-<pre class="brush: js">for (<var>variable</var> in <var>object</var>)
-  statement</pre>
+<pre class="brush: js">for (<var>variable</var> in <var>object</var>) {
+  <var>statement</var>
+}
+</pre>
 
 <dl>
   <dt><code><var>variable</var></code></dt>