Fix typo in coerce_forced_unit docstring

[edaniels]

No description provided.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pnkfelix (or someone else) soon.

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[GuillaumeGomez]

Thanks!

@bors: r+ rollup

[bors]

📌 Commit 3bf1ba7 has been approved by GuillaumeGomez

[bors]

⌛ Testing commit 3bf1ba7 with merge ed16b0a...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: GuillaumeGomez
Pushing ed16b0a to master...