syscall: correct parameter name of Setgid on linux

Change parameter name from uid to gid, to fix an obvious copy-paste
error.

Change-Id: Iba13a45c87fde9625b82976a7d7901af4b705230
Reviewed-on: https://go-review.googlesource.com/16474Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/syscall_linux.go

@@ -881,7 +881,7 @@ func Setuid(uid int) (err error) {
 	return EOPNOTSUPP
 }
 
-func Setgid(uid int) (err error) {
+func Setgid(gid int) (err error) {
 	return EOPNOTSUPP
 }