cmd/gc: add the word 'archive' to the documentation for -pack

The term "package file" is unclear.

R=iant, dave
CC=golang-codereviews
https://golang.org/cl/54760044

src/cmd/gc/doc.go

@@ -38,7 +38,7 @@ Flags:
 	-o file
 		output file, default file.6 for 6g, etc.
 	-pack
-		write a package file rather than an object file
+		write an archive file rather than an object file
 	-e
 		normally the compiler quits after 10 errors; -e prints all errors
 	-p path