Small bug in `HOW_TO_UPDATE.md`

Hi,

In the document, the 
```sh
    cp ../example/libspconv/CMakeLists.txt .
```
should be
```sh
    cp ../example/libspconv/spconv/CMakeLists.txt .
```

`example/libspconv/CMakeLists.txt` seems to be for executable project, while `example/libspconv/spconv/CMakeLists.txt` is to build & install the spconv library.   