Update manual

Author: lehecht

resources/views/manual/tutorials/videos/shortcuts.blade.php

@@ -95,6 +95,10 @@
                     <td><kbd>l</kbd></td>
                     <td>Select tool to attach labels to existing annotations</td>
                 </tr>
+                <tr>
+                    <td><kbd>o</kbd></td>
+                    <td>Toggle the annotation opactiy between 0.0 and 1.0</td>
+                </tr>
                 <tr>
                     <td><kbd>Shift</kbd>+<kbd>l</kbd></td>
                     <td>Select tool to swap labels of existing annotations</td>