temp/megapixels-purism: update url to point to millipixels repo (MR 3640)

Repository was moved.

temp/megapixels-purism/APKBUILD

 # Purism's fork of megapixels, for the Librem 5
 pkgname=megapixels-purism
 pkgver=0.19.0
-pkgrel=2
+pkgrel=3
 pkgdesc="GTK+3 camera app for mobile devices"
-url="https://source.puri.sm/Librem5/megapixels"
+url="https://source.puri.sm/Librem5/millipixels"
 arch="all"
 license="GPL-3.0-only"
 # Required by postprocess.sh