lguest: fix paths in comments

After commit 07fe997, lguest tool has already moved from
Documentation/virtual/lguest/ to tools/lguest/.

Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/tools/lguest/lguest.txt b/tools/lguest/lguest.txt
index 7203ace..06e1f46 100644
--- a/tools/lguest/lguest.txt
+++ b/tools/lguest/lguest.txt
@@ -70,7 +70,7 @@
 
 - Run an lguest as root:
 
-      Documentation/virtual/lguest/lguest 64 vmlinux --tunnet=192.168.19.1 \
+      tools/lguest/lguest 64 vmlinux --tunnet=192.168.19.1 \
         --block=rootfile root=/dev/vda
 
    Explanation: