t0410: make test description clearer

t0410: make test description clearer

Commit 9a4c507886 (t0410: test fetching from many promisor remotes,
2019-06-25) adds some tests that demonstrate not the automatic fetching
of missing objects, but the direct fetching from another promisor remote
(configured explicitly in one test and implicitly via --filter on the
"git fetch" CLI invocation in the other test) - thus demonstrating
support for multiple promisor remotes, as described in the commit
message.

Change the test descriptions accordingly to make this clearer.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>