Update getLatestValidatedLedgerSequence docs

[keefertaylor]

High Level Overview of Change

Suggested change in documentation for more precise meaning.

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Tests (You added tests for code that already exists, or your new feature included in this PR)
• [x] Documentation Updates
• [ ] Release

[codecov[bot]]

Codecov Report

Merging #219 into master will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #219   +/-   ##
=======================================
  Coverage   88.03%   88.03%           
=======================================
  Files          42       42           
  Lines         861      861           
=======================================
  Hits          758      758           
  Misses        103      103           

Impacted Files	Coverage Δ
XpringKit/XRP/DefaultXRPClient.swift	97.16% <100.00%> (ø)

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