RFR: 8196287: Update src/java.desktop/share/legal/libpng.md for libpng 1.6.34

[Phil Race]

bug: https://bugs.openjdk.java.net/browse/JDK-8196287
webrev: http://cr.openjdk.java.net/~prr/8196287/

legal doc update for JDK 10

-phil.