keynav: use correct `ExecStart` command
keynav: use correct `ExecStart` command

When the change requested in

  https://github.com/rycee/home-manager/pull/1082#discussion_r392715440

was applied, the service `ExecStart` attribute was not updated to use
`pkgs.keynav`.

Fixes #1177
PR #1184

rycee/home-manager,rycee/home-manager,rycee/home-manager