Fix java tracer resource sampling example

[cecile75]

What does this PR do? What is the motivation?

Fix java tracer resource sampling example using system property:

When
-Ddd.trace.sampling.rules='[{\"service\": \"my-service\", \"resource\": \"GET /checkout\", \"sample_rate\":1},{\"service\": \"my-service\", \"sample_rate\":0.2}]' is set, the sampling rule is actually not applied to the spans and an log error is written
ERROR datadog.trace.agent.common.sampling.TraceSamplingRules - Couldn't parse Trace Sampling Rules from JSON:

The correct configuration is
-Ddd.trace.sampling.rules='[{"service": "my-service", "resource": "GET /checkout", "sample_rate":1},{"service": "my-service", "sample_rate":0.2}]'

Merge instructions

Merge readiness:

• [ x] Ready for merge

Additional notes

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✅ Documentation Team Review

The documentation team has approved this pull request. Thank you for your contribution!

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Modified Files

• https://docs-staging.datadoghq.com/cecile75/fixjavaresourcesamplingexample/tracing/trace_pipeline/ingestion_mechanisms