mupdf: correct license meta information

The website of MuPDF says that MuPDF is licensed under the terms of the GNU 
Affero General Public License. However, I didn't see which version of that
license they mean.

A clear statement that MuPDF is licensed under the terms of AGPL >= 3 is 
included in the README file of their Git repository:

    git://git.ghostscript.com/mupdf.git
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Ingo Blechschmidt 2017-10-29 17:19:18 +01:00 committed by GitHub
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commit 3afcba3e0a
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@ -103,7 +103,7 @@ stdenv.mkDerivation rec {
homepage = http://mupdf.com;
repositories.git = git://git.ghostscript.com/mupdf.git;
description = "Lightweight PDF, XPS, and E-book viewer and toolkit written in portable C";
license = licenses.gpl3Plus;
license = licenses.agpl3Plus;
maintainers = with maintainers; [ viric vrthra fpletz ];
platforms = platforms.linux;
};