Fix grammatical error in ReadMe

MaggieKimani1 · web-flow · commit b2544025fe63 · 2023-09-26T17:06:15.000+03:00
diff --git a/README.md b/README.md
@@ -73,7 +73,7 @@ var document = new OpenApiDocument
 };
 ```
 
-Reading and writing a OpenAPI description
+Reading and writing an OpenAPI description
 
 ```C#
 var httpClient = new HttpClient
