Fix grammar in README.md
# Jib - Containerize your Gradle Java project
-Jib is [Gradle](https://gradle.org/) plugin for building Docker and [OCI](https://github.com/opencontainers/image-spec) images for your Java applications.
+Jib is a [Gradle](https://gradle.org/) plugin for building Docker and [OCI](https://github.com/opencontainers/image-spec) images for your Java applications.
For information about the project, see the [Jib project README](../README.md).
For the Maven plugin, see the [jib-maven-plugin project](../jib-maven-plugin).
