Add allow_files to cookbook example for using a binary.
Add allow_files to cookbook example for using a binary.

The example implies that this works for an existing binary, but if the rule is a file then it doesn't work. This changes it so that users will know that the allow_files option exists and they likely want it in this case.

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Reviewed-on: https://github.com/bazelbuild/bazel/pull/892
MOS_MIGRATED_REVID=114849501

bazelbuild/bazel-website,bazelbuild/bazel-website,davidzchen/bazel-website,davidzchen/bazel-website,davidzchen/bazel-website,davidzchen/bazel-website,bazelbuild/bazel-website