fix: Use correct conversion from JSON to Material (#754)

Two lines in MaterialJsonConverter.cpp of Json plugins showed the following compile error:
call of overloaded ‘Material(const value_type&)’ is ambiguous

Plugins/Json/src/MaterialJsonConverter.cpp

@@ -49,7 +49,7 @@ void Acts::to_json(nlohmann::json& j, const MaterialSlab& t) {
 }
 
 void Acts::from_json(const nlohmann::json& j, MaterialSlab& t) {
-  Material mat(j.at("material"));
+  Material mat(j["material"].get<Material>());
   t = Acts::MaterialSlab(mat, j.at("thickness").get<float>());
 }
 
@@ -276,7 +276,7 @@ void Acts::from_json(const nlohmann::json& j, volumeMaterialPointer& material) {
     }
     if (key == Acts::jsonKey().datakey and not value.empty()) {
       for (const auto& bin : value) {
-        Acts::Material mat(bin);
+        Acts::Material mat(bin.get<Acts::Material>());
         mmat.push_back(mat);
       }
     }
@@ -339,4 +339,4 @@ void Acts::from_json(const nlohmann::json& j, volumeMaterialPointer& material) {
                                                     bUtility);
     return;
   }
-}
+}