Fix 1 occurrence of when-expression-in-for-loop-to-when-keyword

Use the `#:when` keyword instead of `when` to reduce loop body indentation.

Author: resyntax-ci[bot]

scribble-test/tests/scribble/markdown.rkt

@@ -40,11 +40,9 @@
           (define (contents file)
             (regexp-replace #rx"\n+$" (file->string file) ""))
           (define undefineds (build-markdown-doc src-file "gen.md"))
-          (for ([u (in-list undefineds)])
-            (when (eq? 'tech (car u))
-              (test #:failure-message
-                    (format "undefined tech: ~e" u)
-                    #f)))
+          (for ([u (in-list undefineds)]
+                #:when (eq? 'tech (car u)))
+            (test #:failure-message (format "undefined tech: ~e" u) #f))
           (test #:failure-message
                 (format
                  "mismatch for: \"~a\", expected text in: \"~a\", got:\n~a"