added MeshCollider to RoadMeshCreator.cs V2

[NorthropBtwo]

improved Version of https://github.com/SebLague/Path-Creator/pull/19

Thanks for your feedback. I did the following changes in my code:

• I changed my code to reassign MeshCollider.sharedMesh instead if deleting the collider
• My additions do not add another Transform.Find
• That the users has to match the name string of the mesh holder is the problem of the original version. I don’t think that this is a real issue since this object is auto-generated.