Fix grammar.

README.md

 * [Drest](https://github.com/leedavis81/drest) - A library for exposing Doctrine entities as REST resource endpoints.
 * [HAL](https://github.com/blongden/hal) - A Hypertext Application Language (HAL) builder library.
 * [Hateoas](https://github.com/willdurand/Hateoas) - A HATEOAS REST web service library.
-* [Jane](https://github.com/janephp/janephp/) - A OpenApi client generator, with validation support
+* [Jane](https://github.com/janephp/janephp/) - An OpenApi client generator with validation support.
 * [Negotiation](https://github.com/willdurand/Negotiation) - A content negotiation library.
 * [Restler](https://github.com/Luracast/Restler) - A lightweight framework to expose PHP methods as RESTful web API.
 * [wsdl2phpgenerator](https://github.com/wsdl2phpgenerator/wsdl2phpgenerator) - A tool to generate PHP classes from SOAP WSDL files.