Polish "Use caps in docker file example for consistency"

See gh-19029

spring-boot-project/spring-boot-docs/src/main/asciidoc/deployment.adoc

@@ -58,10 +58,11 @@ Assuming the above `Dockerfile` is the current directory, your docker image can
 
 [indent=0]
 ----
-	docker build --build-arg appjar=path/to/myapp.jar .
+	docker build --build-arg APPJAR=path/to/myapp.jar .
 ----
 
 
+
 [[cloud-deployment]]
 == Deploying to the Cloud
 Spring Boot's executable jars are ready-made for most popular cloud PaaS (Platform-as-a-Service) providers.