Replace non-existant python.install.package with `python.install.pa…

…th` (#11301)

This confused me initially because I was looking in https://docs.readthedocs.io/en/stable/config-file/v2.html#packages for `python.install.package`, which simply doesn't exist.

Instead, this should probably be `python.install.path` which _is_ a required key (#11300), and defaults to `pip`.

docs/user/config-file/index.rst

@@ -128,7 +128,7 @@ you should define the Python version in ``build.tools.python``.
 The ``python`` key contains a list of sub-keys,
 specifying the requirements to install.
 
-- Use ``python.install.package`` to install the project itself as a Python package using pip
+- Use ``python.install.path`` to install the project itself as a Python package using pip
 - Use ``python.install.requirements`` to install packages from a requirements file
 - Use ``build.jobs`` to install packages using Poetry or PDM