[8.x](backport #5712) Fix typo in float format for APM_SAMPLING_RATE

[mergify]

What does this PR do?

Fix float format specification for APM_SAMPLING_RATE

Why is it important?

Agent will not start because apm.Tracer will error out when a badly formatted sampling rate is passed as environment variable

Checklist

• My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in ./changelog/fragments using the changelog tool
• [ ] I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

• Closes APMConfig SamplingRate binary float formatting breaks apm-agent-go tracer instrumentation #5711

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

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This is an automatic backport of pull request #5712 done by [Mergify](https://mergify.com).

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube