cmd/go: fix test broken by incorrect comments

I accidentally used // instead of #.

Change-Id: I2c9b9d40dd83994ce80fc837e8d992d3807f3e24
Reviewed-on: https://go-review.googlesource.com/c/go/+/213659
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

src/cmd/go/testdata/script/build_import_comment.txt

-// TODO: add a go.mod file and test with GO111MODULE explicitly on and off.
-// We only report the 'expects import' error when modules are disabled.
-// Do we report comment parse errors or conflicts in module mode? We shouldn't.
+# TODO: add a go.mod file and test with GO111MODULE explicitly on and off.
+# We only report the 'expects import' error when modules are disabled.
+# Do we report comment parse errors or conflicts in module mode? We shouldn't.
 
 # Import comment matches
 go build -n works.go