File not found for pg_dumps examples

The link to the dump files are broken in the sample session: https://github.com/GoogleCloudPlatform/spanner-migration-tool/blob/master/docs/poc/postgres.md#sample-dump-files
Cart.pg_dump;
Singers.pg_dump.

Open docs/poc/postgres.md at the “Sample dump files” section and inspect the links for Cart.pg_dump and Singers.pg_dump. Update the references so both dump files resolve, then verify the links from the rendered documentation.

Written by the indexing model from the issue text.