match.rs: Formatting: Each branch on separate line

Putting each branch on a new line and terminating each branch with a
comma is consistent with the other examples in this source file, and
less confusing. The comma isn't strictly necessary when the previous
branch ends with a "}" but it's more consistent.

I experimented with a more comprehensive check for code formatting
throughout this tutorial by running `rustc --pretty` in the makefile,
but `rustc --pretty` is a bit too idiosyncratic: It forces a newline
before each comment, it sometimes puts spaces before commas, and it adds
unnecessary quoting like "ain\'t".
Merge pull request #19 from drothlis/match-formatting

match.rs: Formatting: Each branch on separate line