Fix example

Running example as is fails with: type 'depset' is not iterable

Closes #10333.

PiperOrigin-RevId: 283332651

site/docs/skylark/aspects.md

@@ -84,7 +84,7 @@ def _print_aspect_impl(target, ctx):
         # Iterate through the files that make up the sources and
         # print their paths.
         for src in ctx.rule.attr.srcs:
-            for f in src.files:
+            for f in src.files.to_list():
                 print(f.path)
     return []