Reference to incorrect env var "SENTRY_ORGANIZATION_SUBDOMAIN"

https://docs.getodk.org/central-upgrade/ references env var `SENTRY_ORGANIZATION_SUBDOMAIN`, but I can't find any history of this env var in the `odk-central` or `odk-central-backend` repos.

It's likely this should always have read `SENTRY_ORG_SUBDOMAIN`.

Ref:

* https://github.com/search?q=repo%3Agetodk%2Fdocs%20SENTRY_ORGANIZATION_SUBDOMAIN&type=code
* https://github.com/getodk/docs/blob/00982474d4e9916b7fb065587851fedd81ef7126/docs/central-upgrade.rst#L578

Open docs/central-upgrade.rst at the referenced section around line 578 and compare the documented variable with the linked repository search results. Replace the incorrect SENTRY_ORGANIZATION_SUBDOMAIN reference with the confirmed name, then verify the old name no longer appears in the documentation.

Written by the indexing model from the issue text.