gui: ToneDetect: Note it uses the Goertzel algorithm

Note in the GUI help text that the ToneDetect uses the
Goertzel algorithm.

Signed-off-by: Graham Whaley <graham.whaley@gmail.com>

gui/index.html

 <script type="text/x-red" data-help-name="AudioAnalyzeToneDetect">
 	<h3>Summary</h3>
 	<div class=tooltipinfo>
-	<p>Detect the level of a single tone</p>
+	<p>Detect the level of a single tone.</p>
+	<p>Uses the
+	<a href="https://en.wikipedia.org/wiki/Goertzel_algorithm" target="_blank">Goertzel algorithm</a>
+	.</p>
 	</div>
 	<h3>Audio Connections</h3>
 	<table class=doc align=center cellpadding=3>