BF: DOCS: fix slash for windows in conf.py

[brunobeltran]

PR Summary

Single line fix to remove a hard-coded slash that was preventing the docs from building on windows in Anaconda.

PR Checklist

• [N/A] Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• [N/A] New features are documented, with examples if plot related.
• [N/A] Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• [N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[tacaswell]

Anyone can merge on green.

[brunobeltran]

Some extension seems to have changed under us? Doc build works locally, but is failing on CI.

[jklymak]

rebase - fix is on master