rhashtable-test: Fix max_size parameter description

Looks like a simple copy'n'paste error.

Fixes: 1aa661f5 ("rhashtable-test: Measure time to insert, remove & traverse entries")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>