docs(proto): specify time type in vesting proto

[robert-zaremba]

Description

Updates documentation to specify time type for vesting accounts.

Context: I've been creating new vesting accounts and had to go back reading code make sure about the exact type of the end / start time.

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• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
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• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
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• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[JeancarloBarrios]

LGTM

[sonarqubecloud]

[Cosmos SDK] Kudos, SonarCloud Quality Gate passed!   

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