Fix an additional space in help generated

[soumeh01]

Problem:

In the scenario when command line args don't have any options and have only positional args. The help generated shows an extra space when we try to set custom help to empty string in order to overwrite the default [OPTION...] string in the help.

cxxopts::Options options{"test", ""};
std::vector<std::string> positional;
options
  .positional_help("<posArg1>...<posArgN>")
  .custom_help("")
  .add_options()
    ("positional", "", cxxopts::value<std::vector<std::string>>(positional));

options.parse_positional({"positional"});
std::vector<const char*> arg = {"pos1", "pos2", "pos3"};
auto r = o.parse(arg.size(), arg.data());
cout<<options.help();

Current generated help

Usage:
  test  <posArg1>...<posArgN>

Note: there are 2 spaces between test and <posArg1>

Expected help

Usage:
  test <posArg1>...<posArgN>

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This change is 

[jarro2783]

Thanks.