Fix incorrect channel being fitted by the Gaussian spot fitter.

The channel specified by the users in the Spot Gaussian fitter action
was not properly considered. I had forgotten to make a 1-based to
0-based conversion for the channel index we pass to ImgLib2....
Noticed by Thomas Sabate.

Author: tinevez

src/main/java/fiji/plugin/trackmate/action/fit/SpotFitterController.java

@@ -125,7 +125,7 @@ public void run()
 				{
 					final ImagePlus imp = trackmate.getSettings().imp;
 					// 1-based to 0-based.
-					final int channel = gui.getSelectedChannel();
+					final int channel = gui.getSelectedChannel() - 1;
 					final int index = gui.getSelectedFitIndex();
 					final SpotFitter fitter;
 					// Stupid and harsh: