Fix cursor moving down when selection exist. Solves (zyedidia#3087)

Previously `CursorDown` function called `Deselect` with a wrong
argument which lead to the situation when cursor was moved to the
start instead of the end of the selection

Signed-off-by: Yevhen Babiichuk (DustDFG) <dfgdust@gmail.com>

internal/action/actions.go

@@ -176,7 +176,7 @@ func (h *BufPane) CursorUp() bool {
 
 // CursorDown moves the cursor down
 func (h *BufPane) CursorDown() bool {
-	h.Cursor.Deselect(true)
+	h.Cursor.Deselect(false)
 	h.MoveCursorDown(1)
 	h.Relocate()
 	return true