main/ncurses: upgrade to 6.2_p20200509

change source url because mirror doesn't work now,
from the mailing list announce:
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Patches for ncurses 6.2 can be found at
       ftp://ftp.invisible-island.net/ncurses/6.2

The hosting provider for this site made some "update" which breaks my
process for mirroring (I'll resume mirroring when I'm able):
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8 jobs for !7792 with ncurses-6.2-p20200509 in 3 minutes and 29 seconds
detached
Status Job ID Name Coverage
  Lint
passed #116732
docker-alpine x86_64
lint

00:00:29

 
  Build
passed #116737
docker-alpine ci-build aarch64
build-aarch64

00:01:29

passed #116738
docker-alpine ci-build armv7
build-armv7

00:01:49

passed #116736
docker-alpine ci-build ppc64le
build-ppc64le

00:01:52

passed #116783
s390x ci-build docker-alpine
build-s390x

00:01:04

passed #116734
docker-alpine ci-build x86
build-x86

00:01:05

passed #116733
docker-alpine ci-build x86_64
build-x86_64

00:01:11

failed #116735
docker-alpine ci-build s390x
build-s390x

02:00:01