update wasm build instructions
With the new WebAssembly features pushed recently, vtk-wasm can be built with more options using vtk/master so it may be more relevant to encourage the user to use git version with new modules on.
With the new WebAssembly features pushed recently, vtk-wasm can be built with more options using vtk/master so it may be more relevant to encourage the user to use git version with new modules on.