docs: edit release note for schema change feature flag

Release note (sql change): This is an empty commit meant to correct
a mistake in a merged release note in #57040. The original release
note indicates that a database administrator should toggle this
feature flag on and off using
SET CLUSTER SETTING feature.schemachange.enabled, but there
should be a '_' character so that the cluster setting would be:
SET CLUSTER SETTING feature.schema_change.enabled.

Below is the full original release note, with the typo fixed.

Release note (sql change): Adds a feature flag via cluster setting
for all schema change-related features. If a user attempts
to use these features while they are disabled, an error indicating
that the database administrator has disabled the feature is
surfaced.

Example usage for the database administrator:
SET CLUSTER SETTING feature.schema_change.enabled = FALSE;
SET CLUSTER SETTING feature.schema_change.enabled = TRUE;