Use HTTPS in YAML exmaple

See gh-16316

Author: wilkinsona

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -558,20 +558,20 @@ For example, the following YAML document:
 ----
 	environments:
 		dev:
-			url: http://dev.example.com
+			url: https://dev.example.com
 			name: Developer Setup
 		prod:
-			url: http://another.example.com
+			url: https://another.example.com
 			name: My Cool App
 ----
 
 Would be transformed into these properties:
 
 [source,properties,indent=0]
 ----
-	environments.dev.url=http://dev.example.com
+	environments.dev.url=https://dev.example.com
 	environments.dev.name=Developer Setup
-	environments.prod.url=http://another.example.com
+	environments.prod.url=https://another.example.com
 	environments.prod.name=My Cool App
 ----