OPENNLP-1273: Fix JavaDoc error for Java 11 builds (#370)

diff --git a/pom.xml b/pom.xml
index 38493b2..1a4e0a8 100644
--- a/pom.xml
+++ b/pom.xml
@@ -336,6 +336,7 @@
 							<show>public</show>
 							<quiet>false</quiet>
 							<use>false</use> <!-- Speeds up the build of the javadocs -->
+							<source>8</source>
 						</configuration>
 					</execution>
 				</executions>