Update tank material pointing to deleted copy of PBRLighting
In the process of cutting the next alpha release, I ran into a crash with the SimplePBRTest example.
This tank material was previously changed to test the new modular pbr shaders when the shaders were in a separate directory for preliminary testing last year.
But now they were deleted (and have replaced the main PBR shader) so this test crashes with an assetNotFoundException, and just needed updated to point back to Common/MatDefs/Light/PBRLighting.j3md
Ryan McDonough