docs: persistence.md (consistent name for postgresql)

[seanorama]

What changed?

Minor docs fixes.

Made consistent naming, as it was postgres in some places and postgresql in others. The full name helps with finding in searches.

And fixing cases from PostgresQL to PostgreSQL.

Why?

Fix docs.

How did you test it?

Docs only.

Potential risks

Nothing, except maybe invalid markdown.

Release notes

None.

Documentation Changes

None in cadence-docs.

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