DOCS-3443 - clarification re: user for db.copyDatabase() with --auth

Perhaps s/executed/called/ then? Otherwise your sentence seems fine with the prepositional phrases swapped - "you must specify the credentials of a user present in the source database [when calling] |copydb|." which keeps the 'of a user' more closely modifying 'credentials.' I know you love your second person replacements of passive dependent clauses. =)