Readme: replace OpenAPI badge (#3077)

There seems to be an issue at shields.io.
For several days the badge is gray and not operational, no one cares.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **Documentation**
* Enhanced OpenAPI specification badge display in README with improved
rendering for better visual presentation and documentation clarity.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: RobinTail

README.md

@@ -3,8 +3,8 @@
 ![logo](https://raw.githubusercontent.com/RobinTail/express-zod-api/master/logo.svg)
 
 ![CI](https://github.com/RobinTail/express-zod-api/actions/workflows/node.js.yml/badge.svg)
-![OpenAPI](https://img.shields.io/swagger/valid/3.0?specUrl=https%3A%2F%2Fraw.githubusercontent.com%2FRobinTail%2Fexpress-zod-api%2Fmaster%2Fexample%2Fexample.documentation.yaml&label=OpenAPI)
 [![coverage](https://coveralls.io/repos/github/RobinTail/express-zod-api/badge.svg)](https://coveralls.io/github/RobinTail/express-zod-api)
+<img alt="OpenAPI" src="https://validator.swagger.io/validator/?url=https%3A%2F%2Fraw.githubusercontent.com%2FRobinTail%2Fexpress-zod-api%2Fmaster%2Fexample%2Fexample.documentation.yaml" height="20" />
 
 ![downloads](https://img.shields.io/npm/dw/express-zod-api.svg)
 ![npm release](https://img.shields.io/npm/v/express-zod-api.svg?color=green25&label=latest)