Fix placeholder syntax in external config docs

Signed-off-by: Doehyun Baek <doehyunbaek@gmail.com>

Author: doehyunbaek

documentation/spring-boot-docs/src/docs/antora/modules/reference/pages/features/external-config.adoc

@@ -859,7 +859,7 @@ Default Values defined in configuration properties are not reflected in the java
 In the examples above, the `enabled` property of `MyProperties` bound to `my.service` is `false` by default.
 
 However,  `my.service.enabled` is not available in the `Environment` with a value of `false` if no such property is set by the user.
-Concretely, this prevents you to use `@Value(${"my.service.enabled"})` or `my.service.enabled` as a placeholder in configuration properties without explicitly providing a default.
+Concretely, this prevents you to use `@Value("${my.service.enabled}")` or `my.service.enabled` as a placeholder in configuration properties without explicitly providing a default.
 If you need to query the javadoc:org.springframework.core.env.Environment[] for that property, for instance in a javadoc:org.springframework.context.annotation.Condition[] implementation, the default needs to be provided as well.