Fix thymeleaf-layout-dialect.version in doc

Version 2.0.0 had serval bugs which broke compatibility to older
templates, so the guide should refer the fixed version instead.

Closes gh-6991

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

@@ -1465,7 +1465,7 @@ for example:
 ----
 	<properties>
 		<thymeleaf.version>3.0.0.RELEASE</thymeleaf.version>
-		<thymeleaf-layout-dialect.version>2.0.0</thymeleaf-layout-dialect.version>
+		<thymeleaf-layout-dialect.version>2.0.3</thymeleaf-layout-dialect.version>
 	</dependency>
 ----