syscall: fix comment in mkall_windows.bat

src/pkg -> src

LGTM=alex.brainman
R=alex.brainman
CC=golang-codereviews
https://golang.org/cl/139400043

src/syscall/mkall_windows.bat

@@ -4,7 +4,7 @@
 @echo off
 
 if exist mkall.sh goto dirok
-echo mkall_windows.bat must be run from src\pkg\syscall directory
+echo mkall_windows.bat must be run from src\syscall directory
 goto :end
 :dirok