Add case sensitive note to AudioPlaySerialflashRaw documentation

gui/list.html

 		is used to access the flash chip.  You can also use SerialFlash's functions
 		to access the stored files, or add data to the flash chip.
 	</p>
+	<p>File names are case sensitive with SerialFlash.  If your sound does
+		not play, use <b>File &gt; Examples &gt; SerialFlash &gt; ListFiles</b> to
+		check the exact file names stored in the flash memory chip.
 </script>
 <script type="text/x-red" data-template-name="AudioPlaySerialflashRaw">
 	<div class="form-row">