Update CONTRIBUTING.md

Author: thinkingserious

CONTRIBUTING.md

@@ -87,7 +87,7 @@ source ./sendgrid.env
 ```
 
 ```bash
-mvn package
+./gradelew build
 cd ../examples
 javac -classpath ./commons-logging-1.2.jar:./httpcore-4.4.4.jar:./httpclient-4.5.2.jar:./jackson-databind-2.7.3.jar:./jackson-annotations-2.7.0.jar:./jackson-core-2.7.3.jar:../target/http-1.0-SNAPSHOT.jar:. Example.java && java -classpath ./commons-logging-1.2.jar:./httpcore-4.4.4.jar:./httpclient-4.5.2.jar:./jackson-databind-2.7.3.jar:./jackson-annotations-2.7.0.jar:./jackson-core-2.7.3.jar:../target/http-1.0-SNAPSHOT.jar:. Example
 ```
@@ -197,4 +197,4 @@ Please run your code through:
 7. [Open a Pull Request](https://help.github.com/articles/using-pull-requests/)
     with a clear title and description against the `master` branch. All tests must be passing before we will review the PR.
 
-If you have any additional questions, please feel free to [email](mailto:[email protected]) us or create an issue in this repo.
+If you have any additional questions, please feel free to [email](mailto:[email protected]) us or create an issue in this repo.