platform-overview: Tweak licence page to mention LGPLv2.1+ over LGPLv2.1

LGPLv2.1= is a very odd licence to choose, and almost all projects go
with LGPLv2.1+ instead. For consistency with the text below which
mentions GPLv2+, add a ‘+’ to the LGPL sentence.

https://bugzilla.gnome.org/show_bug.cgi?id=376123