test: Fix buffer size option listing in run --help

test/run

@@ -10,7 +10,7 @@ usage() {
     echo
     echo "Interesting options:"
     echo "  --tool_exec=    Test against a different bash executable."
-    echo "  --buffer_size   Change expect match buffer size from our default of 20000 bytes."
+    echo "  --buffer-size   Change expect match buffer size from our default of 20000 bytes."
     echo "  --debug         Create a dbg.log in the test directory with detailed expect match information."
     echo "  --timeout       Change expect timeout from the default of 10 seconds."
     echo "  --debug-xtrace  Create an xtrace.log in the test directory with set -x output."