Fix typo in README

[torinthiel]

Fix an incorrectly named attribute in usage example.

Before creating my pull-request, I ensure:

• [x] Documentation is written
• [ ] Unit tests are written and green
• [x] I've cleaned my WIP or meaningless commits
• [ ] Examples are up-to-date

In order to validate the pull request, I do:

• I hope this is a genuine typo, but hopefully so, given the examples use _vaultpass in this place

[torinthiel]

Ok, actually this a bad PR, but the fact that the example uses vault_path and right after that points to examples with vault_pass is a bit confusing.