A grammatical error in the README.md file

[DaryaYuk]

Describe the bug
In the README.md file, there is a grammatical error

To Reproduce
Read the README.md file
https://github.com/microsoft/OpenAPI.NET/blob/vnext/README.md

Steps to reproduce the current behavior:

Expected behavior
The article a should be replaced with an

Screenshots/Code Snippets

Additional context