Update ADK doc according to issue #1227 - 11 - OpenAPI tool execution

[adk-bot]

11. Update OpenAPI tool execution description (httpx)

Doc file: docs/tools-custom/openapi-tools.md

Current state:

Execution: ... It handles authentication (if configured) and executes the API call using the requests library.

Proposed Change:

Execution: ... It handles authentication (if configured) and executes the API call asynchronously using the httpx library (previously requests).

Reasoning:
RestApiTool has been updated to use httpx for asynchronous execution, improving performance.

Reference: src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py