Update the external documentation URL to use HTTPS.
diff --git a/build.properties b/build.properties
index 7bf5ad9..dfed9c1 100644
--- a/build.properties
+++ b/build.properties
@@ -27,7 +27,7 @@
 jar_dir = ${build_dir}/jar
 doc_dir = ${build_dir}/doc
 
-extdoc_url = http://docs.oracle.com/javase/9/docs/api
+extdoc_url = https://docs.oracle.com/javase/9/docs/api
 extdoc_dir = extdoc
 
 pom_template = maven/pom_template.xml