Remove leading :: from paths in doc examples

[Rantanen]

Noted some pre-2018 path syntax in the doc examples, for example:
https://doc.rust-lang.org/std/process/fn.exit.html

fn main() {
    ::std::process::exit(match run_app() {
       Ok(_) => 0,
       ...

Couldn't find an existing issue on this (then again, "::" makes for an annoying thing to search for) so if there is already something fixing this and/or there's a reason to not fix it, just close this PR.

(Also fixed indentation in the process::exit() docs)

[rust-highfive]

r? @withoutboats

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[Centril]

Note that a leading :: is merely redundant in these cases, not wrong.

Thanks tho! r? @Centril @bors r+ rollup

[bors]

📌 Commit 040d88d has been approved by Centril

[Rantanen]

I thought those were deprecated with 2018 - but I guess that affected only the ::local_mod::something() paths. Oh well!

Paths starting with :: aren't mentioned in the book either (or alternatively I'm blind) so I guess it makes sense to avoid them in the examples.

[Centril]

@Rantanen In the 2018 edition, a path starting with :: always starts from the extern prelude and refers to an external crate. Most of the time such disambiguation is unnecessary however so the change in this PR is towards the more idiomatic.