Fix typo `Assert4J` -> `AssertJ`

See gh-22948

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

@@ -7468,7 +7468,7 @@ TIP: If multiple auto-configurations have to be defined, there is no need to ord
 
 Each test can use the runner to represent a particular use case.
 For instance, the sample below invokes a user configuration (`UserConfiguration`) and checks that the auto-configuration backs off properly.
-Invoking `run` provides a callback context that can be used with `Assert4J`.
+Invoking `run` provides a callback context that can be used with `AssertJ`.
 
 [source,java,indent=0]
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