docs: fix typo

[ghost]

Summary

This PR fixes some typos as the followings:

• were -> was (docs/understanding-results.md)
• mask -> masks (docs/variability.md)
• failing audit -> failing audit (docs/architecture.md)

Related Issues/PRs

N/A

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[connorjclark]

all but one of these changes LGTM.

thanks!

[googlebot]

CLAs look good, thanks!

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[patrickhulce]

LGTM

[brendankenny]

thanks!

[exterkamp]

LGTM, thanks for finding these!

addressed by author :)