correct on_failure param docstring description

[Mark Walle (mark-walle)]

No description provided.

[Coveralls (coveralls)]

Coverage remained the same at 80.482% when pulling fac823d on mark-walle:patch-1 into 4550310 on snowplow:master.

[Snowplow CLA bot (snowplowcla)]

Thanks for your pull request. Is this your first contribution to a Snowplow open source project? Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[Mark Walle (mark-walle)]

I signed it!

…

On Tue, Mar 26, 2019, 01:09 Snowplow CLA bot, ***@***.***> wrote: Thanks for your pull request. Is this your first contribution to a Snowplow open source project? Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA). 📝 Please visit https://github.com/snowplow/snowplow/wiki/CLA to learn more and sign. Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#217 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ADUx_LaDVBhoHedxiXkZiSAQln2Kpyezks5vadWcgaJpZM4cKcUZ> .

[Snowplow CLA bot (snowplowcla)]

Confirmed! Mark Walle (@mark-walle) has signed the Individual Contributor License Agreement. Thanks so much

[Ben Fradet (BenFradet)]

Thanks 👍

[Oguzhan Unlu (oguzhanunlu)]

Thanks for the contribution Mark Walle (@mark-walle) ! It is cherry-picked to the release branch at #224 .