qht-bench: document -p flag

Which we forgot to do in bd224fce60 ("qht-bench: add -p flag
to precompute hash values", 2018-09-26).

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tests/qht-bench.c

@@ -72,6 +72,7 @@ static const char commands_string[] =
     " -n = number of threads\n"
     "\n"
     " -o = offset at which keys start\n"
+    " -p = precompute hashes\n"
     "\n"
     " -g = set -s,-k,-K,-l,-r to the same value\n"
     " -s = initial size hint\n"