Fix port in application.properties (#73)

The `PORT` env var is provided by up itself, however (at least in my testing) this is case sensitive.

Author: mindstorms6

oss/java-maven-springboot/src/main/resources/application.properties

@@ -1,3 +1,3 @@
 spring.main.bannerMode=off
 spring.application.name=demo
-server.port=${port:3000}
+server.port=${PORT:3000}