Clarify "source" mesh in Using MultiMeshInstance (#4334)

I found the original sentence confusing because the word "mesh" is used twice, referring to different nodes. 

> One node is used as the target, the mesh that you want to place multiple meshes
on. 

This target mesh is referred to in the tools and later on in the docs as the surface mesh, so I figure we might as well call it that right away - 

> One node is used as the target, the **surface** mesh that you want to place multiple meshes
on.

tutorials/3d/using_multi_mesh_instance.rst

@@ -25,7 +25,7 @@ Setting up the nodes
 The basic setup requires three nodes: the MultiMeshInstance node
 and two MeshInstance nodes.
 
-One node is used as the target, the mesh that you want to place multiple meshes
+One node is used as the target, the surface mesh that you want to place multiple meshes
 on. In the tree example, this would be the landscape.
 
 The other node is used as the source, the mesh that you want to have duplicated.