Fix target_info metric name in PrometheusHttpServerBuilder javadoc

[thswlsqls]

No issue: trivial javadoc typo, submitting directly.

Description

• PrometheusHttpServerBuilder.setTargetInfoMetricEnabled documents the metric as otel_target_info, but the exported name is target_info.
• Otel2PrometheusConverter.makeTargetInfo() hardcodes MetricMetadata.builder().name("target"), which the Prometheus client exposes as target_info; no code path adds an otel_ prefix.
• PrometheusMetricReaderBuilder.setTargetInfoMetricEnabled, which this method delegates to, already documents it as target_info.
• The name matches the Prometheus compatibility spec, which defines target_info.
• Introduced in Prom exporter update #7934, which added this method.

Testing done

• Docs-only, test-exempt: no test added — existing tests already assert target_info output.
• ./gradlew :exporters:prometheus:check — 202 tests + 1 testJpms test passed.
• No apidiff or CHANGELOG entry: exporters/prometheus is alpha (not tracked by japicmp) and nothing user-facing changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.60%. Comparing base (b86f040) to head (8ec933d).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8628   +/-   ##
=========================================
  Coverage     91.60%   91.60%           
  Complexity    10343    10343           
=========================================
  Files          1013     1013           
  Lines         27352    27352           
  Branches       3215     3215           
=========================================
  Hits          25057    25057           
  Misses         1567     1567           
  Partials        728      728           

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