Reword prereq in downstream SQL Server connector doc

I somehow missed fixing the issue that gmorling@redhat.com referenced in his comment on PR 2256: https://github.com/debezium/debezium/pull/2256#discussion_r607731060

Update the text of the second bullet in the Prerequisites (L1662) to clarify that SQL Server does not run the Debezium connector. Instead of stating that SQL server is set up to run a connectors, say that it is set "to work with a {prodname} connector."