Fix documentation for SGTL lineInLevel (swap channels)

gui/index.html

 14: 0.29 Volts p-p
 15: 0.24 Volts p-p
 </pre>
-	<p class=func><span class=keyword>lineInLevel</span>(left, right);</p>
+	<p class=func><span class=keyword>lineInLevel</span>(right, left);</p>
 	<p class=desc>Adjust the sensitivity of the line-level inputs, with different
 		settings for left and right.  The same 15 settings are available.
 	</p>