Fix dead API reference in rationale.md try-with-resources example

[thswlsqls]

Description

• The "Span not Closeable" example in docs/rationale.md calls TracingContextUtils.currentContextWith(span). Neither that class nor the currentContextWith method exists in the codebase (confirmed via grep), so copy-pasting the example does not compile.
• Since Span extends ImplicitContextKeyed, span.makeCurrent() returns a Scope — the current API that fits try-with-resources directly.
• The sibling doc docs/sdk-configuration-design.md already uses the try (Scope ignored = span.makeCurrent()) pattern; this PR aligns the example with it.

Testing done

• Docs-only change (1 line in docs/rationale.md). No behavior change → no new tests, not a Gradle build target.
• Confirmed in context/.../ImplicitContextKeyed.java that span.makeCurrent() returns Scope, matching the example's try (Scope scope = ...).
• No apidiff / CHANGELOG change (not a public API or user-facing behavior change).

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: thswlsqls / name: Eunbin Son (d65cea0)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (824334c) to head (d65cea0).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8488   +/-   ##
=========================================
  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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[thswlsqls]

/easycla

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