docs: fix doubled backslashes in Windows pip config path

[aroh3006]

What was wrong

Two pages give the location of pip's Windows configuration file, and both write the path with two backslashes per separator inside an inline literal:

• docs/reference/environment-variables.rst — "Alternative: Configuration Files"
• docs/how-to/corporate-environments.rst — "Configuration Files"

reStructuredText does not treat backslashes as escape characters inside inline literals, so the doubled separators are rendered verbatim. Readers are shown a path with doubled separators, which is not a valid Windows path — anyone copying it to find or create their pip.ini gets the wrong location.

This is visible on the published docs today, on both stable and latest:

• https://build.pypa.io/en/stable/reference/environment-variables.html
• https://build.pypa.io/en/latest/how-to/corporate-environments.html

pip's own documentation gives this location with single separators (https://pip.pypa.io/en/stable/topics/configuration/), and the surrounding docs are already consistent with that — the TEMP example a few sections up in the same reference page uses single separators.

How it was verified

• Rendered the exact source lines through docutils before and after: the current source renders with doubled separators, the fixed source renders the correct path.
• Built the full docs with the project's own settings, sphinx-build -W -n -b html docs, which succeeded, and diffed the generated HTML for both pages — each now renders the correct single-separator path.
• Cross-checked against the live published pages (linked above) and against pip's documentation.
• Confirmed git grep finds no other occurrence of the doubled form in the repository.

What changed

One line in each of the two files, changing the doubled separators in the inline literal to single ones. Nothing else — no prose, formatting, or unrelated files.

Validation performed

sphinx-build -W -n completed with no warnings or errors. I could not run the full tox -e docs environment locally (it pins base_python = "3.14"; I have 3.12), so proselint did not run — but this change touches no prose, only the contents of an inline literal.

Happy to fold in the second file separately if you would rather keep the how-to and reference changes apart.

Before / after on Read the Docs

The Read the Docs preview for this PR renders the correct path on both pages, which can be compared directly against the currently published docs:

	Reference page	How-to page
Published today	environment-variables	corporate-environments
This PR	environment-variables	corporate-environments

[henryiii]

This is way too much for a minor docs mixup. :)

[aroh3006]

makes sense. I'll keep the explanations shorter.

[henryiii]

Thanks!