Make the path of the config directory better understandable

Currently, the documentation relates to the subdirectory for external
configurations as `/config`. Read as UNIX path, the leading slash
could be read as reference to the root directory `/`.

This commit updates the documentation to use 'config/' instead.

See gh-32224

Author: obfischer

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

@@ -108,8 +108,8 @@ Spring Boot will automatically find and load `application.properties` and `appli
 .. The classpath `/config` package
 . From the current directory
 .. The current directory
-.. The `/config` subdirectory in the current directory
-.. Immediate child directories of the `/config` subdirectory
+.. The `config/` subdirectory in the current directory
+.. Immediate child directories of the `config/` subdirectory
 
 The list is ordered by precedence (with values from lower items overriding earlier ones).
 Documents from the loaded files are added as `PropertySources` to the Spring `Environment`.