docs(CONTRIBUTING): clarify Java/JDK toolchain; capitalize terms; tidy code block

[deshkaustubh]

Summary
Update CONTRIBUTING.md to improve clarity and consistency. Docs-only; no code or CI changes.

Changes (line refs based on commit 9da5ff0)

• L5: Reword environment sentence
  "The SDK requires Java 8, but development requires JDK 21 for the Kotlin toolchain."
  -> "The SDK requires Java 8 for runtime, but development requires JDK 21 for the Kotlin toolchain."
  Permalink: https://github.com/deshkaustubh/openai-java/blob/169195c547693bff17fb97b243bb4bbd81650f66/CONTRIBUTING.md#L5

• L187: Capitalize “GitHub Action”
  "### GitHub action]"
  -> "### GitHub Action]"
  Permalink: https://github.com/deshkaustubh/openai-java/blob/169195c547693bff17fb97b243bb4bbd81650f66/CONTRIBUTING.md#L187

• L206: Capitalize “Gradle” in section heading
  "### Available gradle tasks"
  -> "### Available Gradle tasks"
  Permalink: https://github.com/deshkaustubh/openai-java/blob/169195c547693bff17fb97b243bb4bbd81650f66/CONTRIBUTING.md#L206

• L106: Remove stray blank line inside the code block under “Then run the tests”
  Before:

  $ ./scripts/test
  

  After:

  $ ./scripts/test

  Permalink: https://github.com/deshkaustubh/openai-java/blob/169195c547693bff17fb97b243bb4bbd81650f66/CONTRIBUTING.md#L106

Rationale

• Clarifies Java runtime vs. development toolchain
• Ensures consistent capitalization
• Cleans up minor Markdown formatting

Scope/Impact

• Documentation only
• No behavioral, build, or release changes

Checklist

• Markdown renders correctly
• Docs-only change; no tests required
• No changelog/release note needed

[deshkaustubh]

Hi team 👋 Just checking in on this PR — All visible checks have passed.
Would love a quick review or any feedback when someone gets a chance. Happy to iterate if needed 🙌
Thanks for your time!