fixed wrong usage of document.isElementAt

Signed-off-by: Florian Thienel <florian@thienel.org>
diff --git a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/widget/VexWidgetImpl.java b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/widget/VexWidgetImpl.java
index 68a35bb..db17b4d 100644
--- a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/widget/VexWidgetImpl.java
+++ b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/widget/VexWidgetImpl.java
@@ -359,7 +359,7 @@
 				deleteSelection();
 			} else {
 				offset--;
-				if (doc.isElementAt(offset)) {
+				if (!doc.isElementAt(offset)) {
 					this.moveTo(offset, false);
 					this.moveTo(offset + 1, true);
 					deleteSelection();