improve Antiforgery docs

[DeagleGross]

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aspnetcore/security/anti-request-forgery.md	aspnetcore/security/anti-request-forgery

[guardrex]

Supplying a quick grammatical/style manual review.

[guardrex]

Thanks @DeagleGross! ... Will let you pull the trigger when you're ready.

For future reference, the docs try to stick with present tense. I changed "need" to "require" to avoid 'personification of inanimate objects' (an app). If you need help with XREFs, you can use the little XREF gen tool that I created at https://dotnet.github.io/blazor-samples/, which is also a sample app for how to host a Blazor WASM app on GH Pages.