Fix notebook `intermediate/indexing/why-an-index.ipynb`

Open intermediate/indexing/why-an-index.ipynb and run it from top to bottom, focusing on Cell In[16] where periodic.sel(angle=q) produces a one-element angle dimension. Check the array-input indexing behavior and update the notebook so the plotted values match q's 721 elements; rerunning the full notebook without the reported dimension error is done.

This notebook (looking at 54df4e2fc01e51905210793f7aa01df0d87be86b ) doesn't run top to bottom

Cell In[16], [line 7](vscode-notebook-cell:?execution_count=16&line=7)
      3 periodic_temp = periodic.sel(angle=q)
      4 
      5 fig, ax = plt.subplots(figsize=(9, 4))
      6 ax.scatter(sensor_fold, metal_ring.values, color="0.3", s=45, zorder=5, label="the sensors")
----> [7](vscode-notebook-cell:?execution_count=16&line=7) ax.plot(q, periodic_temp, color="tab:green", lw=2.5, label="PeriodicIndex .sel")
      8 ax.plot(q, default_temp, color="tab:red", lw=2, label="default index")
      9 ax.axvline(0, color="0.6", ls=":", lw=1)
     10 ax.set_xlabel("query angle (°)")

File ~/coding/repos/xarray-tutorial/.pixi/envs/default/lib/python3.14/site-packages/matplotlib/axes/_axes.py:1792, in Axes.plot(self, scalex, scaley, data, *args, **kwargs)
   1549 """
   1550 Plot y versus x as lines and/or markers.
   1551 
   (...)   1789 (``'green'``) or hex strings (``'#008000'``).
   1790 """
   1791 kwargs = cbook.normalize_kwargs(kwargs, mlines.Line2D)
-> [1792](https://file+.vscode-resource.vscode-cdn.net/Users/Hodgs004/coding/repos/xarray-tutorial/intermediate/indexing/~/coding/repos/xarray-tutorial/.pixi/envs/default/lib/python3.14/site-packages/matplotlib/axes/_axes.py:1792) lines = [*self._get_lines(self, *args, data=data, **kwargs)]
   1793 for line in lines:
   1794     self.add_line(line)

File ~/coding/repos/xarray-tutorial/.pixi/envs/default/lib/python3.14/site-packages/matplotlib/axes/_base.py:331, in _process_plot_var_args.__call__(self, axes, data, return_kwargs, *args, **kwargs)
    329     this += args[0],
...
    511 if x.ndim > 2 or y.ndim > 2:
    512     raise ValueError(f"x and y can be no greater than 2D, but have "
    513                      f"shapes {x.shape} and {y.shape}")

ValueError: x and y must have same first dimension, but have shapes (721,) and (1,)

I don't think the index is working as expected when angle is an array input.

periodic_temp = periodic.sel(angle=q) # `q.shape` is `(721,)`
# but `periodic_temp.sizes` is `Frozen({'angle': 1})`

cc @ianhi