Fix a typo in error messages in std::fs tests

[Kintaro]

Just a small correction to fix a typo in an error message in std::fs tests

[rust-highfive]

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

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[pitdicker]

I thought the first letter was missing so both upper- and lowercase messages where matched. But all the other test don't use this, and it works ok.

[GuillaumeGomez]

The first letter is really missing. Thanks @Kintaro!

r=me @steveklabnik

[apasel422]

@bors: r=GuillaumeGomez

[bors]

📌 Commit 0c05953 has been approved by GuillaumeGomez

[bors]

⌛ Testing commit 0c05953 with merge e90307d...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-32cross-opt, auto-linux-64-cargotest, auto-linux-64-cross-armhf, auto-linux-64-cross-armsf, auto-linux-64-cross-freebsd, auto-linux-64-cross-netbsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild