Fixes a typo in the pgbackrest docs

[gregblake]

Checklist:

• [ ] Have you added an explanation of what your changes do and why you'd like them to be included?
• [x] Have you updated or added documentation for the change, as applicable?
• [ ] Have you tested your changes on all related environments with successful results, as applicable?

Type of Changes:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

What is the current behavior? (link to any open issues here)

N/A

What is the new behavior (if this is a feature change)?

N/A

Other information:

N/A