Update content.html

This URL https://unicode-table.com/ has changed to https://symbl.cc/

web/content.html

@@ -150,7 +150,7 @@ <h3>How to use the generated fonts in LVGL?</h3>
 
     <h3>Useful notes</h3>
     <ul class="ul-space">
-        <li><strong>Unicode table</strong> to pick letters: <a href="https://unicode-table.com/" target="_blank">https://unicode-table.com/</a></li>
+        <li><strong>Unicode table</strong> to pick letters: <a href="https://symbl.cc/" target="_blank">https://symbl.cc/</a></li>
         <li><strong>Unicode ranges</strong> <a href="http://jrgraphix.net/research/unicode.php" target="_blank">http://jrgraphix.net/research/unicode.php</a></li>
         <li><strong>A pixel perfect font</strong>: <a href="https://sourceforge.net/projects/terminus-font/">Terminus</a>. </li>
         <li><strong>Examples for two-tone or colored fonts</strong>: <a href="https://github.com/google/material-design-icons/blob/master/font/MaterialIconsTwoTone-Regular.otf">Material Design Icons (two-tone version)</a>,