pscan: fix a typo in help text

diff --git a/include/usage.h b/include/usage.h
index 7918b24..54b6a4d 100644
--- a/include/usage.h
+++ b/include/usage.h
@@ -3024,7 +3024,7 @@
        "Scan a host, print all open ports\n" \
      "\nOptions:" \
      "\n	-c	Show closed ports too" \
-     "\n	-c	Show blocked ports too" \
+     "\n	-b	Show blocked ports too" \
      "\n	-p	Scan from this port (default 1)" \
      "\n	-P	Scan up to this port (default 1024)" \
      "\n	-t	Timeout (default 5000 ms)" \