check_http: s/--header_string/--header-string/

The name of the new option is "--header-string".

plugins/check_http.c

@@ -207,7 +207,7 @@ process_arguments (int argc, char **argv)
     {"port", required_argument, 0, 'p'},
     {"authorization", required_argument, 0, 'a'},
     {"proxy_authorization", required_argument, 0, 'b'},
-    {"header_string", required_argument, 0, 'd'},
+    {"header-string", required_argument, 0, 'd'},
     {"string", required_argument, 0, 's'},
     {"expect", required_argument, 0, 'e'},
     {"regex", required_argument, 0, 'r'},