Add the -s flag to the suggested rustup invocation

[ghost]

curl's progress meter would otherwise interfere with sudo's password prompt.

[rust-highfive]

r? @steveklabnik

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[ghost]

Here's what I'm talking about:

$ curl -L https://static.rust-lang.org/rustup.sh | sudo sh
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 13083  100 13083    0     0  74812      0 --:--:-- --:--:-- --:--:-- 75189Password:

See the Password prompt at the end? I didn't, for about 20 seconds. :-)

[brson]

I know this is unrelated, but do you mind adding -sf, not just -s?

-f produces an error if the page isn't found, and sending 404 garbage through a sudo shell is bad.

[ghost]

As I was editing the file from GitHub's UI, I ended up creating the branch in the official rust-lang repo. Closing this PR to reopen with the right branch.