Correct spelling of ‘--separator’ option in README document

[Ben Finney]

The README document reports a mis-spelling of the --separator option, in the Changelog section:

- **1.15.0** Added `--seperator` argument, German wordlist (GPL 3.0, thanks to @anlx-sw)

This should be corrected to match the English word “separator” and the --separator option.

[Steven Tobin]

Good catch, fixed in latest releases!