fix: grammar error for `apmFlushDeadline` description

[duncanista]

What does this PR do?

Updates grammar error from #85

Motivation

DataDog/datadog-ci#816

Testing Guidelines

n/a

Additional Notes

Types of changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [x] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [x] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [x] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog

[codecov-commenter]

Codecov Report

Merging #86 (2fcfa29) into master (ea21073) will increase coverage by 0.28%. The diff coverage is 100.00%.

:exclamation: Current head 2fcfa29 differs from pull request most recent head cfdb461. Consider uploading reports for the commit cfdb461 to get more accurate results

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   93.01%   93.29%   +0.28%     
==========================================
  Files           7        7              
  Lines         501      522      +21     
  Branches      117      124       +7     
==========================================
+ Hits          466      487      +21     
  Misses         26       26              
  Partials        9        9              

Impacted Files	Coverage Δ
serverless/src/env.ts	97.32% <100.00%> (+0.61%)	:arrow_up:

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