selftests/run_kselftest.sh: Fix help string for --per-test-log

This is documented as --per_test_log but the argument is actually
--per-test-log.

Link: https://lore.kernel.org/r/20241220-per-test-log-v1-1-de5afe69fdf4@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: bjackman

tools/testing/selftests/run_kselftest.sh

@@ -21,7 +21,7 @@ usage()
 	cat <<EOF
 Usage: $0 [OPTIONS]
   -s | --summary		Print summary with detailed log in output.log (conflict with -p)
-  -p | --per_test_log		Print test log in /tmp with each test name (conflict with -s)
+  -p | --per-test-log		Print test log in /tmp with each test name (conflict with -s)
   -t | --test COLLECTION:TEST	Run TEST from COLLECTION
   -c | --collection COLLECTION	Run all tests from COLLECTION
   -l | --list			List the available collection:test entries