fix typo in W0_Z_SI definition

This fixes a typo in `src/picongpu/include/simulation_defines/param/laserConfig.param` that creates a non-intuitive compile error:

```
/net/cns/projects/HPL/xray/pausch/PIConGPU/picongpu/src/picongpu/include/fields/FieldE.kernel(87): error: identifier "_ZN8picongpu15laserWavepacket4W0_ZE" is undefined
```