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>
diff --git a/2016/asm.slide b/2016/asm.slide
index 65ca53e..d75231d 100644
--- a/2016/asm.slide
+++ b/2016/asm.slide
@@ -221,7 +221,7 @@
 
 Look at the generated assembly for this simple program:
 
-.code asm/add.go.txt
+.code asm/add.go
 
 For each architecture, with some noise edited out: