docs: remove duplicate 'a' in AuthorizationPolicy authentication note

[mrchatam]

Fixes #2154

Remove duplicate article in the AuthorizationPolicy note:

• before: "require a a variety of forms of authentication"
• after: "require a variety of forms of authentication"

Docs-only, one-line change.

[travisbeckham]

Thanks for catching this!

It appears that this typo is also present in docs from 2.12 through 2.19, as well as the 2-edge doc.

Would you be able to fix these as well?

[travisbeckham]

Fixed via #2157