fix the /v2/_catalog n parameter description
fix the /v2/_catalog n parameter description

If `n` is not present only 100 entries returned

Signed-off-by: lisong <c49bf9ea15db2960edb6a66d2b137280c9cac54e@cdsunrise.net>

thaJeztah/distribution,docker/distribution,thaJeztah/distribution,docker/distribution