README.md:  Fix incorrect --data-dir=~/.keyid-foo syntax


Usually expanding the ~ syntax is the role of the shell.  Bash will not do this when it occurs in the middle of an argument; e.g.

    $ echo --data-dir=~/.KeyID-foo
    --data-dir=~/.KeyID-foo

The syntax should be `--data-dir=$HOME/.KeyID-foo`.

Using the `$HOME` syntax is only necessary if the program relies on the shell for expanding the tilde.  If there is code in the BitShares Toolkit to parse the argument and run `~/.KeyID-foo` through some library which expands the tilde syntax, then this pull request is unnecessary (but harmless).