Fix grammar in contributing docs

This commit fixes an instance of odd comma placement in the contributing
docs.

Author: jasontedor

CONTRIBUTING.md

@@ -95,7 +95,7 @@ Contributing to the Elasticsearch codebase
 JDK 10 is required to build Elasticsearch. You must have a JDK 10 installation
 with the environment variable `JAVA_HOME` referencing the path to Java home for
 your JDK 10 installation. By default, tests use the same runtime as `JAVA_HOME`.
-However, since Elasticsearch, supports JDK 8 the build supports compiling with
+However, since Elasticsearch supports JDK 8, the build supports compiling with
 JDK 10 and testing on a JDK 8 runtime; to do this, set `RUNTIME_JAVA_HOME`
 pointing to the Java home of a JDK 8 installation. Note that this mechanism can
 be used to test against other JDKs as well, this is not only limited to JDK 8.