update(doc): fix invalid HTML (#9899)

Author: LuxDie

docs/app/partials/layout-introduction.tmpl.html

@@ -201,7 +201,7 @@ <h3>
       <tr>
         <td>layout-gt-lg</td>
         <td>flex-gt-lg</td>
-        <td>width &gt;= <b>1920/b>px</td>
+        <td>width &gt;= <b>1920</b>px</td>
       </tr>
       <tr>
         <td>layout-xl</td>