more helpful hint for touch devices

index.html

@@ -277,7 +277,11 @@
 <div class="hint">
     <p>Use a spacebar or arrow keys to navigate</p>
 </div>
-
+<script>
+if ("ontouchstart" in document.documentElement) { 
+    document.querySelector(".hint").innerHTML = "<p>Tap on the left or right to navigate</p>";
+}
+</script>
 
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