MAINT: Fix grammar in user_guide/scale.rst
Follow-up to:

https://github.com/pandas-dev/pandas/pull/29811
@pandas-dev/pandas-core : This PR is a simple doc change, but we seem to have some Azure failures here that were present on `master` for a good several commits it seems. It's an excel related example that is failing (and there was a minor openpyxl version change), opened https://github.com/pandas-dev/pandas/issues/29854 for this thanks @gfyoung