Document using ThreadLocal#remove instead of ThreadLocal#set(null)

See gh-32874

Author: Seungpang

framework-docs/modules/ROOT/pages/core/aop-api/targetsource.adoc

@@ -222,7 +222,7 @@ incorrectly using them in multi-threaded and multi-classloader environments. You
 should always consider wrapping a `ThreadLocal` in some other class and never directly use
 the `ThreadLocal` itself (except in the wrapper class). Also, you should
 always remember to correctly set and unset (where the latter simply involves a call to
-`ThreadLocal.set(null)`) the resource local to the thread. Unsetting should be done in
+`ThreadLocal.remove()`) the resource local to the thread. Unsetting should be done in
 any case, since not unsetting it might result in problematic behavior. Spring's
 `ThreadLocal` support does this for you and should always be considered in favor of using
 `ThreadLocal` instances without other proper handling code.