Add example to the free-vars docs.

Thanks to Christophe Scholliers.

pkgs/racket-doc/syntax/scribblings/free-vars.scrbl

@@ -1,5 +1,5 @@
 #lang scribble/doc
-@(require "common.rkt" (for-label syntax/free-vars))
+@(require "common.rkt" (for-label syntax/free-vars) scribble/example)
 
 @title[#:tag "free-vars"]{Computing the Free Variables of an Expression}
 
@@ -23,3 +23,19 @@ sub-expressions before extracting identifiers. The default
 
 If @racket[module-bound?] is non-false, the list of free variables also
 includes free module-bound identifiers.
+
+@examples[
+(require (for-syntax racket/base syntax/parse syntax/free-vars))
+(define-syntax (print-body-free-vars stx)
+  (syntax-parse stx
+    #:literals (lambda)
+    [(_ (~and lam (lambda (a ...) b ...)))
+     (define expanded-body (local-expand #'lam 'expression '()))
+     (syntax-parse expanded-body
+       #:literals (#%plain-lambda)
+       [(#%plain-lambda (arg ...) body)
+        (displayln (free-vars #'body))
+        expanded-body])]))
+
+(lambda (x) (print-body-free-vars (lambda (y) x)))
+]