Updated question mark image in sonar gui docs.

Author: torgeirwl

docs/img/Sonar-3D-15-question-mark-solid-transparent-20x20.png

@@ -2,7 +2,7 @@
 
 The GUI is designed to be user friendly with a clean an intuitive set of controls.
 
-When you first start the Sonar 3D-15 GUI you will be greeted with a view showing the 3D point cloud and various settings. To get familiar with the interface, click the ![?](../img/Sonar-3D-15-question-mark-transparent-50x50.png) icon (help overlay).
+When you first start the Sonar 3D-15 GUI you will be greeted with a view showing the 3D point cloud and various settings. To get familiar with the interface, click the ![?](../img/Sonar-3D-15-question-mark-solid-transparent-20x20.png) icon (help overlay).
 The 3D view can be controlled via mouse and keyboard, or touch controls.