MFH: r445993 www/py-rfc3987: Fix LICENSE_FILE typo

Add missing filename suffix to LICENSE_FILE, without which causes an error at
patch time:

===>  Missing license file for GPLv3+ in /wrkdirs/usr/ports/www/py-rfc3987/work/rfc3987-1.3.7/COPYING
*** Error code 1

Pointyhat: koobs (QA every change!)
Reported by: pkg-fallout

Approved by: ports-secteam (blanket)