Document GPL3 option, and fix a typo

All files in cidToUnicode, nameToUnicode and unicodeMap are available unchanged in "language support packages for Xpdf", downloadable from http://www.xpdfreader.com/download.html. In these tarballs, the READMEs state that:

If this package includes CMap files, they contain their own copyright
notices and distribution conditions.  All other files in the package
are Copyright 2002 Glyph & Cog, LLC, and are licensed under the GNU
General Public License (GPL), version 2 or 3.

AFAIU, there are no CMap files in the named poppler-data directories, so the GPL3 option applies.

A shell script for a reproducible check against the poppler-data 0.4.9 tarball is at https://gitlab.freedesktop.org/-/snippets/1167