[css-fonts-4] Rename system to system-ui per WG resolution <https://lists.w3.org/Archives/Public/www-style/2015Nov/0272>

Author: fantasai

css-fonts-4/Overview.bs

@@ -132,7 +132,7 @@ of predefined CSS generic font families to cover these type styles.</p>
 
 <p>Add new generic font families for:</p>
 <ul>
-<li><code>system</code> font family representing the default UI font on a given platform</li>
+<li><code>system-ui</code> font family representing the default UI font on a given platform</li>
 <li><code>emoji</code> font family for emoji characters</li>
 <li><code>math</code> font family used for rendering mathematical expressions</li>
 <li><code>fangsong</code> font family used for <em>fang song</em> typefaces in Chinese</li>