Doc: wrong symlink documented

it is currently:

```
ln -s /usr/lib64/python3.7/site-packages/_selinux.cpython-37m-x86_64-linux-gnu.so ./lib/python3.7/site-packages/selinux
```

but it must be

```
ln -s /usr/lib64/python3.7/site-packages/selinux ./lib/python3.7/site-packages/selinux
```