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

pkgs/applications/misc/mupdf/default.nix

@@ -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;
   };