docs tests: Fix use of migrate_set_parameter

docs tests: Fix use of migrate_set_parameter

docs/multi-thread-compression.txt uses parameter names with
underscores instead of dashes.  Wrong since day one.

docs/rdma.txt, tests/qemu-iotests/181, and tests/qtest/test-hmp.c are
wrong the same way since commit cbde7be900d2 (v6.0.0).  Hard to see,
as test-hmp doesn't check whether the commands work, and iotest 181
appears to be unaffected.

Fixes: 263170e679df (docs: Add a doc about multiple thread compression)
Fixes: cbde7be900d2 (migrate: remove QMP/HMP commands for speed, downtime and cache size)
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>