[css-color-5] update color-adjust example

Author: svgeesus

css-color-5/Overview.bs

@@ -557,7 +557,7 @@ Adjusting colors: the ''color-adjust'' function {#coloradjust}
 		with the lightness being reduced by 20%.
 		The chroma and hue of red are left unchanged.
 
-		<pre class="lang-css">color-adjust(peru lightness(-20%));</pre>
+		<pre class="lang-css">color-adjust(peru lightness -20%);</pre>
 
 		The calculation is as follows:
 		* <span class="swatch" style="--color: peru"></span> peru (#CD853F) is lch(62.2532% 54.0114 63.6769)