toggle instead of always showing the help overlay when ? is pressed

js/reveal.js

@@ -3297,7 +3297,12 @@
 
 		// Check if the pressed key is question mark
 		if( event.shiftKey && event.charCode === 63 ) {
-			showHelp();
+			if( dom.overlay ) {
+				closeOverlay();
+			}
+			else {
+				showHelp( true );
+			}
 		}
 
 	}