Fix documentation example

Previously, the nested private static class would cause an error at
startup stating that it was not accessible when trying to bind the
property from the environment. The nested class should be public.

Closes gh-4323

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

@@ -813,7 +813,7 @@ as `@Valid` to trigger its validation. For example, building upon the above
 
 		// ... getters and setters
 
-		private static class RemoteAddress {
+		public static class RemoteAddress {
 
 			@NotEmpty
 			public String hostname;