Update persistence.rst

[nurzhanzhanuzak]

Fix typo: an -> a

[coveralls]

Pull Request Test Coverage Report for Build 7602536365

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 7599338374:	0.0%
Covered Lines:	5156
Relevant Lines:	5156

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💛 - Coveralls

[johnbywater]

Thank you very much for your attention to the details!