asm: fix asm.slide

add.go.txt did not exist

TBR=bradfitz

Change-Id: If5d9e7c8411d1cb3bdc5a40b72fc4ef21abd7b3b
Reviewed-on: https://go-review.googlesource.com/27570
Reviewed-by: Rob Pike <r@golang.org>
1 file changed