go/build: update docs for GOOS.go change

Forgotten in https://golang.org/cl/147690043/

Update #8838

LGTM=r
R=r
CC=golang-codereviews, rsc
https://golang.org/cl/152220045

src/go/build/doc.go

@@ -108,12 +108,10 @@
 //	*_GOOS
 // 	*_GOARCH
 // 	*_GOOS_GOARCH
-// (example: source_windows_amd64.go) or the literals:
-//	GOOS
-// 	GOARCH
-// (example: windows.go) where GOOS and GOARCH represent any known operating
-// system and architecture values respectively, then the file is considered to
-// have an implicit build constraint requiring those terms.
+// (example: source_windows_amd64.go) where GOOS and GOARCH represent
+// any known operating system and architecture values respectively, then
+// the file is considered to have an implicit build constraint requiring
+// those terms.
 //
 // To keep a file from being considered for the build:
 //