CFE-2915 Align example in docs with bodies that exist in the stdlib

While the example is technically correct, using the body name from the stdlib
will encourage use of existing bodies instead of duplicating them as someone
trying to copy/paste will receive a duplicate definition of body error.

reference/promise-types/files.markdown

@@ -1473,17 +1473,17 @@ object.
 
        "/home/mark/tmp/testcopy"
 
-         delete => tidyfiles,
+         delete => tidy,
          file_select => changed_within_1_year,
          depth_search => recurse("inf");
 
        # Now delete the parent.
 
        "/home/mark/tmp/testcopy"
-         delete => tidyfiles;
+         delete => tidy;
      }
 
-     body delete tidyfiles
+     body delete tidy
      {
      dirlinks => "delete";
      rmdirs   => "true";