package/nmap: fix license

package/nmap: fix license

Commit 78dc1f185ba0c8c9085d44318f72ab172867b147 forgot to update the
license file from COPYING to LICENSE.

Here is an extract of the ChangeLog for Nmap 7.90 [2020-10-03]:

Upgraded the Nmap license form a sort of hacked-up version of GPLv2 to a
cleaner and better organized version (still based on GPLv2) now called
the Nmap Public Source License to avoid confusion. See
https://nmap.org/npsl/ for more details and annotated license text. This
NPSL project was started in 2006 (community discussion here:
https://seclists.org/nmap-dev/2006/q4/126) and then it lost momentum for
7 years until it was restarted in 2013
(https://seclists.org/nmap-dev/2013/q1/399) and then we got distracted
by development again. We still have some ideas for improving the NPSL,
but it's already much better than the current license, so we're applying
NPSL Version 0.92 to the code now and can make improvements later if
needed. This does not change the license of previous Nmap releases.

Fixes:
 - http://autobuild.buildroot.org/results/8cef6a5e99ae341cced405a389346e2faccf6eec

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>