fix xlim error in RWG example
Frequencies are given in Hz, so the correct command would be
```
xlim(100e9, 1300e9)
```
However, this is not really necessary in this case.
This fixes the _empty plot_ in the documentary.
"xlabel('Frequency(GHz)')\n",
"ylabel('Loss (dB/m)')\n",
"title('Loss in Rectangular Waveguide (Au)');\n",
- "xlim(100,1300)"
]
},
{
