Missed backtick fix
auto merge of #868 : IanConnolly/cargo/backtick_fix, r=alexcrichton

19:09 < ianconnolly> hey, i was browsing the cargo code, any particular reason why this string is mixed backticks + straight apostrophes?
                     https://github.com/rust-lang/cargo/blob/master/src/bin/cargo.rs#L157
19:10 < acrichto> ianconnolly: oh I think I missed that, it should probably just be `{}` like the rest of cargo