talks: add content/ element to import paths

CL 141317 moved the talk content in the content/ subdirectory.
This CL follows up on that change by updating import paths accordingly.
After this change, go test golang.org/x/talks/... passes.

Remove some trailing whitespace from content/2016/applicative.slide.

Change-Id: Iccfa77e0d8b5175a4dc0d97983fba09ce912ce50
Reviewed-on: https://go-review.googlesource.com/c/141878
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
14 files changed