tutorial: Add missing struct name

[jhckragh]

Without Point before each { ... } I get the following error when
compiling the code with the compiler from the preliminary tarball made
available on Wednesday:

error: mismatched types: expected `Point` but found `{x: float,y: float}`
(expected class Point but found record)

[nikomatsakis]

Thanks.

[catamorphism]

I un-merged this in master since it was pulled directly to master, and building it against incoming, I get a compile error because the test case now has a comment saying that Point is a structural record type. So I'm leaving it un-merged.

[brson]

I merged this.

[jhckragh]

Doh! Sorry about targeting the pull request at the wrong branch.