Auto merge of #2780 - Boddlnagg:patch-1, r=alexcrichton
Auto merge of #2780 - Boddlnagg:patch-1, r=alexcrichton

Fix `cargo doc --open` on Windows

This fixes #2446. Note that I have not built cargo with this change, but I have tested the functionality in isolation (on Windows 10).

As to the issue itself, I don't know why the previous version didn't work, but `start` is redundant when `cmd /C` is used.

vojtechkral/cargo,achanda/cargo,eminence/cargo,tsurai/cargo,AndiDog/cargo,tshepang/cargo,mark-buer/cargo,rust-lang/cargo,nipunn1313/cargo,tsurai/cargo,brson/cargo,AndiDog/cargo,AndiDog/cargo,rustyhorde/cargo,mark-buer/cargo,sbeckeriv/cargo,vojtechkral/cargo,mark-buer/cargo,tsurai/cargo,tshepang/cargo,nipunn1313/cargo,durka/cargo,vojtechkral/cargo,achanda/cargo,AndiDog/cargo,tsurai/cargo,nipunn1313/cargo,rustyhorde/cargo,durka/cargo,achanda/cargo,sbeckeriv/cargo,tshepang/cargo,sbeckeriv/cargo,tsurai/cargo,rust-lang/cargo,brson/cargo,tshepang/cargo,mark-buer/cargo,durka/cargo,nipunn1313/cargo,rustyhorde/cargo,mark-buer/cargo,brson/cargo,eminence/cargo,vojtechkral/cargo,brson/cargo,eminence/cargo,achanda/cargo,durka/cargo,rust-lang/cargo,brson/cargo,AndiDog/cargo,rust-lang/cargo,eminence/cargo,rustyhorde/cargo,sbeckeriv/cargo