Dockerfile: update golang.org/x/talks/... import path
Dockerfile: update golang.org/x/talks/... import path

CL 141317 moved the talk content in the content/ subdirectory.
This CL follows up on that change by updating an import path accordingly.

It fixes the following build error:

	Step 52/61 : RUN GOOS=nacl GOARCH=amd64p32 go get [...] golang.org/x/talks/2016/applicative/google [...]
	[...]: cannot find package "golang.org/x/talks/2016/applicative/google" in any of:
		/usr/local/go/src/golang.org/x/talks/2016/applicative/google (from $GOROOT)
		/go/src/golang.org/x/talks/2016/applicative/google (from $GOPATH)
	The command [...] returned a non-zero code: 1

Change-Id: I0b711d32023066a8b4a16ceb4cbe2c7b79d66f77
Reviewed-on: https://go-review.googlesource.com/c/144477
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/playground,golang/playground,golang/playground