Use ascii chatacter in HTML file

Author: tompng

lib/rdoc/generator/template/darkfish/_sidebar_toggle.rhtml

@@ -1,3 +1,3 @@
 <button id="navigation-toggle" class="navigation-toggle" aria-label="Toggle sidebar" aria-expanded="true" aria-controls="navigation">
-  <span aria-hidden>☰</span>
+  <span aria-hidden>&#9776;</span>
 </button>