UITableView has a property separatorInset. You can use that to set the insets of the table view separators to zero to let them span the full width of the screen.
[tableView setSeparatorInset:UIEdgeInsetsZero];
Note: If your app is also targeting other iOS versions, you should check for the availability of this property before calling it by doing something like this:
if ([tableView respondsToSelector:@selector(setSeparatorInset:)]) {
[tableView setSeparatorInset:UIEdgeInsetsZero];
}