Fix typo with file endings in example

[goetzk]

Example provided in documentation didn't match the validity checks embedded in vultr.py inventory plugin.

Description

Update example filenames to match in-code validity checks

running ansible-inventory with -vvv shows

host_list declined parsing /[path]/vultr-instances.yaml as it did not pass its verify_file() method
Skipping due to inventory configuration file name mismatch. Valid filename endings: vultr.yaml, vultr.yml, vultr_hosts.yaml, vultr_hosts.yml, vultr_instances.yaml, vultr_instances.yml

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

[resmo]

good catch. Thanks!

[goetzk]

Thanks for the quick merge!