Fixed usage string: a value should not be specified with --del.

$ cat /tmp/test.ini
[test]
a = 123

[test2]
verbose = on

$ crudini --del /tmp/test.ini test2 verbose on
A value should not be specified with --del
A utility for manipulating ini files

Usage: crudini --set [OPTION]...   config_file section   [param] [value]
  or:  crudini --get [OPTION]...   config_file [section] [param]
  or:  crudini --del [OPTION]...   config_file section   [param] [value]
  or:  crudini --merge [OPTION]... config_file [section]
...