Fix --extcap-dlts call in extcap_example.py.

There is regular expression that extracts only the number from
--extcap-interface argument and only that number (as string) is being
passed to extcap_dlts().

Change-Id: I5159f9405a766c1edff792213b2aef72b9a29ba4
Reviewed-on: https://code.wireshark.org/review/4550
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>