Make term "trust store" consistent across the docs

[toraritte]

Please see PR smallstep/docs#260 for context.

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Name of feature:

Term consistency

Pain or issue this feature alleviates:

Makes it possible to find all instances of the term "trust store".

Why is this important to the project (if not answered above):

I find consistency and discoverability of concepts important.

Supporting links/other PRs/issues:

None that I know of.

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[tashian]

just one suggestion here