`extern crate` and `mod` are not easily confused

Remove some statements that used to refer to similarities between `mod` and
`extern mod`, before the latter was renamed to `extern crate`.
Fix reference to "these two traits."

There are actually three traits listed.
Minor spelling/grammar/usage fixes.

Note: "different to" is not exactly incorrect, but "different from" is more
commonly accepted in both US and Commonwealth English, and also more
consistent with other usage within this tutorial.
auto merge of #13270 : mbrubeck/rust/doc-edit, r=alexcrichton

Just some minor tweaks for clarity and correctness.  Please see the commit messages for explanations.