junicode: change license to OFL

Both the project page ([1], third paragraph) and the
documentation ([2], second-to-last paragraph on the last page)
indicate that the font is available under SIL OFL.

[1]: http://junicode.sourceforge.net/
[2]: http://junicode.sourceforge.net/Junicode.pdf

pkgs/data/fonts/junicode/default.nix

@@ -15,6 +15,6 @@ fetchzip {
   meta = {
     homepage = http://junicode.sourceforge.net/;
     description = "A Unicode font for medievalists";
-    license = lib.licenses.gpl2Plus;
+    license = lib.licenses.ofl;
   };
 }