fix: stray LIRInstruction token in SpanBuilder Javadoc

[thswlsqls]

Description

• The public SpanBuilder.setStartTimestamp(long, TimeUnit) Javadoc opens with a stray LIRInstruction. token: <p>LIRInstruction.Use this method to specify an explicit start timestamp.
• LIRInstruction is an unrelated GraalVM compiler class — a copy-paste artifact that renders as broken doc text.
• The sibling overload setStartTimestamp(Instant) has the intended wording (<p>Use this method ...), so the fix is to drop the token.
• Javadoc-only: no behavior or public API change.
• Precedent: docs typo fix Fix Value Javadoc typo #8479 ("Fix Value Javadoc typo").

Testing

• ./gradlew :api:all:check passed (spotlessCheck + ErrorProne/NullAway + jApiCmp + tests).
• Javadoc-only: no test added; no apidiff change (docs/apidiffs/ untouched), no CHANGELOG entry.

[linux-foundation-easycla]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (824334c) to head (d376290).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8482   +/-   ##
=========================================
  Coverage     78.77%   78.77%           
  Complexity     8579     8579           
=========================================
  Files          1009     1009           
  Lines         28993    28993           
  Branches       3599     3599           
=========================================
  Hits          22839    22839           
  Misses         5311     5311           
  Partials        843      843           

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