The commands mentioned in the docs with gzip and gunzip did not prefix
the archives with ".gz" and used inconsistent paths for the archives,
which can be confusing.

Reported-by: Philipp Gramzow
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/161397938841.15451.13129264141285167267@wrigleys.postgresql.org