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 <bradfitz@golang.org>
diff --git a/Dockerfile b/Dockerfile
index 878cf70..646866d 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -183,7 +183,7 @@
     golang.org/x/tour/reader \
     golang.org/x/tour/tree \
     golang.org/x/tour/wc \
-    golang.org/x/talks/2016/applicative/google && \
+    golang.org/x/talks/content/2016/applicative/google && \
     rm -rf $GOPATH/src/golang.org/x/tour/.git && \
     rm -rf $GOPATH/src/golang.org/x/talks/.git