Correct goto-harness documentation to cover the output is a C file

The behaviour of goto-harness is for the out file to be a C file,
containing the harness.

Author: Thomas Kiley

src/goto-harness/goto_harness_parse_options.cpp

@@ -174,7 +174,8 @@ void goto_harness_parse_optionst::help()
     << " goto-harness <in> <out> --harness-function-name <name> --harness-type "
        "<harness-type> [harness options]\n"
     << "\n"
-    << "<in> <out>                 goto binaries to read from / write to\n"
+    << "<in>                       goto binaries to read from\n"
+    << "<out>                      C file to write the harness to\n"
     << "--harness-function-name    the name of the harness function to "
        "generate\n"
     << "--harness-type             one of the harness types listed below\n"