Fix broken spec data model link in ImmutableSummaryData javadoc

[thswlsqls]

Description

• The class javadoc "See:" link pointed to datamodel.md#summary, which 404s; the spec file is data-model.md.
• Fixed the filename to data-model.md, so the link now resolves to the spec's summary section.
• Mirrors the sibling ImmutableHistogramData, which already links to data-model.md#histogram.

Testing done

• Docs-only trivial typo, test-exempt: no test added; behavior, signature, public API, and observable output are unchanged.
• ./gradlew :sdk:metrics:check passed (:sdk:metrics:test 698 tests). Internal package, japicmp-exempt: no apidiff change. No CHANGELOG entry (not user-facing).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.64%. Comparing base (09d6c17) to head (93831f0).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8640   +/-   ##
=========================================
  Coverage     91.64%   91.64%           
  Complexity    10348    10348           
=========================================
  Files          1013     1013           
  Lines         27380    27380           
  Branches       3218     3218           
=========================================
  Hits          25092    25092           
  Misses         1558     1558           
  Partials        730      730           

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