fixed wording

extension/ui/pages/help.html

@@ -522,8 +522,8 @@
 						<br>
 						You can also use the following keyboard shortcuts when removing contents:
 						<ul>
-							<li><code>Tab</code>: select the parent element</li>
-							<li><code>Shift-Tab</code>: select the child element</li>
+							<li><code>Tab</code>: expand the selection</li>
+							<li><code>Shift-Tab</code>: reduce the selection</li>
 							<li><code>Space</code>: remove the selected element</li>
 							<li><code>Ctrl-Z</code>: undo the last removal</li>
 							<li><code>Ctrl-Shift-Z</code>: redo the last removal</li>