Fixed a/an typo in Interpolating Network Sets
`Interpolating Network Sets.ipynb` had the following typo:
a old -> an old
"cell_type": "markdown",
"metadata": {},
"source": [
- "In this example, we are characterizing a old [narda phase shifter 3752](https://nardamiteq.com/docs/119-PHASESHIFTERS.PDF) at 1.5 GHz. \n",
+ "In this example, we are characterizing an old [narda phase shifter 3752](https://nardamiteq.com/docs/119-PHASESHIFTERS.PDF) at 1.5 GHz. \n",
"![narda 3752 phase shifter](phase_shifter_measurements/Narda_3752.jpg) :\n",
"\n",
"In order to deduce the phase shift that one can obtain at this specific frequency, we have measured scattering parameters in the 1-2 GHz band at 19 positions of the phase knob (from 0 to 180). These measurements are loaded into a [NetworkSets](../../tutorials/NetworkSet.ipynb) object:"
