Fix typo in external-config.adoc

See gh-34386

Author: jongwooo

spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/external-config.adoc

@@ -555,7 +555,7 @@ For example, the following specifies that the second document is only active whe
 
 [[features.external-config.encrypting]]
 === Encrypting Properties
-Spring Boot does not provide any built in support for encrypting property values, however, it does provide the hook points necessary to modify values contained in the Spring `Environment`.
+Spring Boot does not provide any built-in support for encrypting property values, however, it does provide the hook points necessary to modify values contained in the Spring `Environment`.
 The `EnvironmentPostProcessor` interface allows you to manipulate the `Environment` before the application starts.
 See <<howto#howto.application.customize-the-environment-or-application-context>> for details.