Change default metallic value to 0, as per the XMLreference doc.

See https://mujoco.readthedocs.io/en/stable/XMLreference.html#asset-material-metallic

PiperOrigin-RevId: 736190166
Change-Id: Ic841df9510aff5df8f5ea3394732ab65b550aa65

src/user/user_init.c

@@ -287,7 +287,7 @@ void mjs_defaultMaterial(mjsMaterial* material) {
   material->texrepeat[0] = material->texrepeat[1] = 1;
   material->specular = 0.5;
   material->shininess = 0.5;
-  material->metallic = 1.;
+  material->metallic = 0.0;
   material->roughness = 1.0;
   material->rgba[0] = material->rgba[1] = material->rgba[2] = material->rgba[3] = 1;
 }