Updated simple lighting default material.

ScriptableRenderPipeline/LightweightPipeline/LWRP/Materials/Lightweight-StandardSimpleLighting.mat

     - _UVSec: 0
     - _ZWrite: 1
     m_Colors:
-    - _Color: {r: 1, g: 1, b: 1, a: 1}
+    - _Color: {r: 0.5, g: 0.5, b: 0.5, a: 1}
-    - _SpecColor: {r: 1, g: 1, b: 1, a: 1}
+    - _SpecColor: {r: 0.5, g: 0.5, b: 0.5, a: 1}