Update README.md

Fixed grammar issue in line 126.

README.md

@@ -123,6 +123,6 @@ Read our [contributing guide](https://ant.design/docs/react/contributing) and le
 
 We welcome all contributions. Please read our [CONTRIBUTING.md](https://github.com/ant-design/ant-design/blob/master/.github/CONTRIBUTING.md) first. You can submit any ideas as [pull requests](https://github.com/ant-design/ant-design/pulls) or as [GitHub issues](https://github.com/ant-design/ant-design/issues). If you'd like to improve code, check out the [Development Instructions](https://github.com/ant-design/ant-design/wiki/Development) and have a good time! :)
 
-If you are collaborator. Please follow our [Pull Request principle](https://github.com/ant-design/ant-design/wiki/PR-principle) to create Pull Request by [collaborator template](https://github.com/ant-design/ant-design/compare?expand=1&template=collaborator.md).
+If you are a collaborator, please follow our [Pull Request principle](https://github.com/ant-design/ant-design/wiki/PR-principle) to create a Pull Request by [collaborator template](https://github.com/ant-design/ant-design/compare?expand=1&template=collaborator.md).
 
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