Remove unnecessary Arc usage in introduction

[aochagavia]

cc @steveklabnik

[rust-highfive]

r? @huonw

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[steveklabnik]

Hmm, the point is to introduce comboing Arc and Mutex together. The examples have gotten a bit out of sync from each other, I think, but I'm not sure this is the right way to address this. Thoughts?

[aochagavia]

I also think it is nice to introduce Arc. However, I think it is difficult to find a trivial example where it makes sense to use it (probably with detached threads).

[huonw]

r? @steveklabnik (transferring reviewership, don't have the bandwidth right now.)

[steveklabnik]

I'm planning on doing more overhauls to this section, but let's just merge this for now, and we'llsee how it ends up.

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit 5a82a81 has been approved by steveklabnik

[bors]

⌛ Testing commit 5a82a81 with merge b41f2df...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt