Fix wording, example with two mapped dimensions does not support NN/HNSW

[jobergum]

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[thomasht86]

I think that the cells in the notebooks will try to get executed during build on readthedocs if they are not executed at commit.
From https://nbsphinx.readthedocs.io/en/0.5.0/pre-executed.html:

Automatically executing notebooks during the Sphinx build process is an important feature of nbsphinx. However, there are a few use cases where pre-executing a notebook and storing the outputs might be preferable. Storing any output will, by default, stop nbsphinx from executing the notebook.

The readthedocs-environment (which is separate from the github runners) does not have access to secrets, so the unexecuted cell below:

from vespa.application import Vespa

app: Vespa = vespa_cloud.deploy()

will likely fail due to missing key. Found this out the hard way myself :D

As for the failed links, it looks like our reference docs were not built correctly on last run for some reason, https://pyvespa.readthedocs.io/en/latest/reference-api.html - I am investigating.