docs(mcp): correct New Relic API key header casing

[nellai-m]

What problem does this PR solve?
The documentation previously used the incorrect API key header casing (api-key).
New Relic expects the header to be Api-Key, and the mismatch could cause authentication issues or confusion for users following the docs.

Additional context / review notes

• Documentation-only change; no functional or code updates.
• Aligns examples with New Relic’s expected header format.
• Helps prevent setup errors when configuring the New Relic MCP server.

Related issues

• N/A

[CLAassistant]

All committers have signed the CLA.

[github-actions]

Hi @nellai-m 👋

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[WriteMayur]

Our team has confirmed that both api-key and Api-Key work correctly across VSCode and Claude Desktop. Hence, I am closing this PR as updates suggested in this PR is not required.

However, we have opened PR #22600 to enhance the JSON structure for better clarity. Thanks!