update plink link
update plink link

Close #134.

The link-to-file-in-repo doesn't work here because it's a submodule, and
GitHub gets confused. Link directly to the project instead, and assume
folks can read enough to understand what submodule is being referenced.

benknoble/Dotfiles,benknoble/Dotfiles,benknoble/Dotfiles,benknoble/Dotfiles,benknoble/Dotfiles