sync/atomic: make BUG comment godoc-compatible

go/doc wants to see text after BUG(uid): on the same line
in order to treat it as an official bug comment.

R=gri
CC=golang-dev
https://golang.org/cl/4250043

src/pkg/sync/atomic/doc.go

@@ -22,8 +22,7 @@
 //
 package atomic
 
-// BUG(rsc):
-// On ARM, the 64-bit functions use instructions unavailable before ARM 11.
+// BUG(rsc): On ARM, the 64-bit functions use instructions unavailable before ARM 11.
 //
 // On x86-32, the 64-bit functions use instructions unavailable before the Pentium.