README: Cleanup additional references to 'stephenfin'
README: Cleanup additional references to 'stephenfin'

The project has moved to the 'getpatchwork' organization on GitHub.
Cleanup any references to the old location.

Signed-off-by: Stephen Finucane <06fa905d7f2aaced6dc72e9511c71a2a51e8aead@that.guru>

stephenfin/git-pw,getpatchwork/git-pw